As I explained before, mainly because there was not - during all of the Middle Ages -a comparable literature or cultural tradition in Rome with respect to Florence.
Anyhow, nowadays the Florentine dialect has partly deviated from its original form and pronunciation, so that in Florence today you can find the Italian language (standard) and the Florentine dialect (similar to Italian, but not identical - especially in pronunciation: e.g. initial c pronounced h in certain conditions....).
And for the 'defense' of the Roman speech, I would like to add that there is a sort of proverb defining which is the very best Italian pronunciation, which sounds ''lingua toscana in bocca romana'' (Tuscan language as spoken by a Roman mouth).
Does this mean Italians look up to the Roman pronunciation? Would you say Roman accent is considered attractive/desirable? Also do Italians think of modern Romans as descendents of ancient Romans?