It might help if Venice (and area) are bigger patrons of the arts, so poets and writers WANT to write in Veneto.
Definitly. The importance of the florentine dialect was determined by the preminence of the many works written during 11th and 12th century.
It should be noted, though, that italian suffered a strange fate: during 16th century the language stopped to evolve: the intellectuals of the time choose to use only the florentine based as "lingua franca", deeming it the best, and to avoid any use of the other dialects.
Italian language, in other words, crystallized around those years, and, even today, if you are fluent in italian, you can read works written during the reinassance with little or no use of the dictionary.