Italian is only one of the various Romance languages, offspringing from Latin (like French, Spanish, Portugues, Romanian etc).
'Italians' first spoke just various dialects - still in the medieval ages sources are written in a great variety of different [native] dialects (or in Latin).
During the 14th century the Tuscan dialect began to predominate, because of the central position of Tuscany in Italy, and because of the aggressive commerce of its most important city, Florence.
Besides that, Florentine culture produced the three literary artists who best summarized Italian thought and feeling of the late Middle Ages and early Renaissance: Dante, Petrarch, and Boccaccio.
Furthermore, Tuscan departs least in morphology and phonology from classical Latin, and it therefore harmonizes best with the Italian traditions of Latin culture.
And than the
Accademia della Crusca was founded in 1583, which has been accepted by Italians as authoritative in Italian linguistic matters.
And from that times onwards ... :wink: