Mozart's higher creativity than Salieri's is discussed in
Therivel's GAM/DP Theory of Personality and Creativity.

Why Mozart and Not Salieri?

The above is the title of chapter 2 of volume 1 of William A. Therivel's The GAM/DP Theory of Personality and Creativity (G stands for genetic endowment, A for assistances of youth, M for misfortunes of youth, DP for division of power, UP for unity of power). For an introduction to the GAM part of the theory click "Introduction to GAM"; for an introduction to the DP part click on "Introduction to DP".
In this website, the reader is also offered a shortcut: The GAM/DP Synopsis and an expanded version, The GAM/DP Summary of volumes 1 through 4.

Hereafter are excerpts from chapter 2 of volume 1 of The GAM/DP Theory of Personality and Creativity:

     In writing about Mozart and Salieri-two composers, contemporaries of each other who lived and worked in Vienna in the 1700s-Sternberg (1988) remarked that "both men are obviously intelligent. But Mozart was far more creative than Salieri, and in the long run this difference has been easily recognized" (p.241).
     Both men suffered major misfortunes of youth. This, sadly, is not uncommon, but Antonio Salieri, in comparison with Mozart, endured stronger misfortunes which were not compensated by strong assistances. Salieri's mother died when he was 12, and soon after his father, who had been "a well-situated merchant who traded in agricultural products" (Braunbehrens, 1992, p.14), "and misfortunes of divers kinds had fallen upon the family, so that the children--six in all--were left almost in bitter poverty. Antonio took refuge with the [older] brother [a monk] in Padua" (Thayer, 1989, p. 28). Salieri's initial musical assistance, before the death of the parents, came from another brother of his, Francesco, who had studied at the Padua conservatory and who was often employed to play the violin at church functions. (Usually, in nonafflicted families, the assistance from brother to brother is neither of the highest caliber nor assimilated gladly).

Mozart
     Mozart's major misfortune of youth was that of paternal failure of character and profession. This is one of the most challenging misfortunes when compensated by quality assistances, as was also the case for Cervantes, Goethe, Beethoven, Dickens, Einstein, Freud, Marx, and Picasso. Papa Mozart-because of his difficult character and less than attractive personality-never had much of a career. He never made it to Kappellmeister, and, to his great bitterness, he remained Vize-Kappellmeister until his death. On the side of the assistances-and it is not a contradiction-Mozart benefited greatly from the concentrated help from his father, who was a first-rate musician: "Leopold Mozart was a supreme teacher who understood how to inspire gifted children to great effort and achievement, instilling a drive for excellence and awakening in them a sense of unlimited devotion to his person and a desire to obtain his approval above all else" (Solomon, 1995, p. 39). "Without the influence of the father, reflected both in the son's submission and resistance to it, Wolfgang would never have achieved the character and the greatness that he did" (Einstein, 1946, p. 5).
     Leopold was both a quality composer and the author of a valuable Treatise on the Fundamental Art of Violin Playing, which went into several editions. As a composer he was "a prolific and competent craftsman . . .[who] wrote in many of the standard genres, both secular and sacred: passions and oratorios, theater pieces, symphonies, serenades, concertos for solo wind instruments, trios and divertimentos . . .It speaks for the workmanship of his music that several incomplete masses by him were until recently attributed to his son" (Solomon, 1995, p. 31). Several of his pieces (e.g., "Sinfonia burlesca," "Sleigh Ride," "Peasant Wedding") are regularly played in concert halls and on the radio.
     In Mozart's case, the misfortune led directly to the assistance: his father, lacking personal professional success, concentrated all his energies on training and helping his son. Then, as an added assistance, Mozart benefited from a very favorable birth-gender order (and gender prejudice) in being the second and last child after his five-years-older sister Nannerl: "Though Leopold was affectionate towards Nannerl, she became chiefly a willing catalyst in the projection of Wolfgang's talents. Wolfgang's precocity changed Leopold's life and almost turned his head. He became the servant of his son, utterly devoted to him" (Hutchings, 1976, p.20). Nannerl's role "was secondary: Mozart was the mainspring of his father's fame, wealth, and standing, the instrument by which his father's unfulfilled career was redeemed; he was the extension of Leopold's own self, the source of his power" (Solomon, 1995, p. 62).
     Lastly, Mozart probably benefited from a superior genetic endowment (for intelligence, stamina, pitch discrimination, memory for tones, finger coordination). Einstein (1946) noted that "in these years [at age 6 to 10] he was very teachable. And whatever his father prescribed he worked at for a time with the greatest industry, so that he seemed to forget everything else, even music, for a certain period. When, for instance, he was learning arithmetic, the table, the chairs, the walls, and even the floor were covered with figures written in chalk" (p. 25). Solomon (1995) related that "Mozart doggedly taught himself to play the violin at the age of six, insinuating himself into a trio rehearsal at home, playing second violin, and then managing the first violin part with wrong and irregular positioning but without ever actually breaking down" (p. 39).
     Solomon (1995) also devoted a whole chapter, "The Zoroastran Riddles," to another aspect of Mozart, the witty intellectual:

     This is exactly the kind of rapid lateral creativity which we would expect in Goethe or Picasso, and which demands a high G.

Salieri (born in 1750, six years before Mozart.)
     In contrast to Mozart, Salieri's advanced musical education began only at the age of 15, first for a short time in Venice, then in Vienna as the adopted pupil of Florian Leopold Gassmann, court ballet and chamber music composer. This assistance was invaluable for young Salieri (see Rice, 1998, pp. 15ff). Still, it was not great, psychologically and professionally speaking, in comparison with that received by Mozart, who from his earliest years benefited not only from the help of his relatives, but also from his city's cultural and musical life, in which his father was an active participant. Salzburg was renowned as the "German Rome," as the seat of the prince-archbishop of the Salzkammergut province and of a university founded in 1622. Salieri, instead, was born and raised in the small, unimportant city of Legnago, situated 27 miles south-west of Verona.
     Lacking sufficient assistances, Salieri could not become a challenged personality. The right assistance is that which gives a youth self-confidence, a strong faith in his or her capabilities and potential. We think immediately of Sigmund Freud, who said: "A man who has been the indisputable favorite of his mother keeps for life the feeling of a conqueror, the confidence of success that often induces success" (quoted by Jones, 1953, p.5). Salieri received a different type of assistance, that which teaches obedience by undermining self-confidence. His patron Gassmann decided one day to teach young Antonio strict obedience and restrained behavior after he had gone on a small unauthorized errand that had turned slightly awkward. No word of reproach came at first from Gassmann, but the next day Salieri heard his patron say to his Italian coachman (vetturino):

     At any moment, young Salieri could be sent back to his old life of poverty. His musical life in Vienna rested in the goodwill of his benefactor and little in himself. The way his protector taught strict obedience undermined Salieri's self-confidence and taught him to fear his master, be he Gassmann or, later, his employer and protector, Emperor Joseph II. Experimenting with something truly new was therefore dangerous, and Salieri avoided it automatically.

     The answer to the title question is that Salieri was a different/conventional personality while Mozart was a challenged personality who, even when asking the opinion of the renowned composer and musical theorist Padre Martini, could not refrain from stating his wonderful philosophy of life: learn, creat, move forward courageously, contribute to others.
     Salieri was not a challenged personality, and cannot be blamed for this. He was a fine person, a fine musician. He did not poison Mozart, nor did he accelerate his death (beyond not going out of his way to favor a first-class competitor). Were it not for the rumors that he was Mozart's arch-enemy, we would not study him, or not more than any other second- or third-rate composer. Still, besides his music, we can be thankful to him for providing a sharp contrast-within the same field of activity, the same times and place of operation-to a full-fledged challenged personality. In summary, in the formula GxAxM, and in comparison with Mozart, Salieri was long on misfortunes, poor on assistances, and probably not as rich in genetic endowment.
     In essence, different GxAxM is at the origin of different personalities (and personality families, e.g. the conventional, the dedicated). A high GxAxM is at the origin of the challenged personalities with their high potential for creativity; important insights come from a war of scripts between those of the individual and those of society; these insights can be converted into valid acts of creation through hard work, perseverance, some external help, and luck.