You know the traveling bard character that Role Playing Games like so much? The closest existing things were french troubadours. Specifically southern french troubadours. They contributed in making the now-extinct Occitan Variety of French is the first post-latin romance language to develop a body of written works.

Southern France was a weird place between the twelfth and fourteenth centuries. A combination of decentralized power structure and strong local aristocracy led to the development of a culture of traveling entertainers who performed and composed performance art in the local language. These artists were called troubadours. The troubadours rapidly began to travel beyond the borders of France: first to Aragon, where people spoke a language very similar to Occitan, and subsequently to Italy. During the crusades, troubadours followed French knights all the way to Greece and the Holy Land, plus it was fashionable for the Occitan upper classes to dabble in songwriting and instrument playing themselves, spreading Occitan performance art all over the Mediterranean.

So Rustichello da Pisa was an Italian-born troubadour, educated in traditional french song and storytelling. Although song and poetry in Italy was mostly composed in Latin, Francophiles in big cities like Naples and Milan enjoyed Occitan performers, and local artists emulated these Occitans. In cosmopolitan mediterranean ports like Pisa, Rustichello's hometown, local crowds would expect "popular" songs and ballads to be performed in Occitan. In addition, like so many artists before and after him, Rustichello picked up odd jobs when money was scarce; traveling around the Western Mediterranean as a sailor. In the Western Mediterranean, Occitan was a universal language understandable by Catalans and Italians; for Rustichello, it made perfect sense to write in Occitan.

The Divine comedy wouldn't be published for another forty years, so Italian literature was still very much ingrained in Latin traditions, and there's a distinct possibility Rustichello didn't even know Latin; he was a performer and a storyteller, not an artist.