I wrote about this topic in this older answer, although I'm more than happy to answer additional questions.

The fundamental reason why Modern Standard Italian is based on the Tuscan dialect is because of the enormous, lasting, and unquestioned popularity of Tuscan literature. However, modern Italian and the Tuscan dialect are not the same thing; in fact, the Italian Language was standardized after Italy's unification rather arbitrarily by a clique circling around Alessandro Manzoni, an influential writer and socialite who after his nomination to the Senate headed the parliamentary committee on language standardization.