Tuscany had one of the largest populations, one of the highest literacy rates, and their accent, like their bread, is very bland and easy to digest. Almost every Italian, even at the time, would have had no trouble understanding a standard Tuscan dialect, whereas Piedmontese, Calabrian, Sicilian, and Venetian are not really mutually intelligible. Also even if Piedmont united the country the province itself had a small population and not as literate as other provinces. Many Italian writers had already established the practice of writing in Tuscan before Cavour launched the campaign of unification.

Further, the Piedmontese didn't want to be seen as an occupying power, their goal was an Italian Kingdom, not a Piedmontese one, if they imposed their dialect and ruled from Turin the union would have never lasted. Garibaldi would have never conquered the south without encouraging Southern peasants and bourgeoise to rebel in his favor. It wasn't just practical in terms of mutual intelligibility, it was also practical in terms of appearances.

To begin with... the Kings of Piedmont didn't even speak the Piedmontese dialect, but French; this was a common occurrence until the 19th-century, and it was only with the proclamation of the new Kingdom of Italy that the language lost its status at Court, for fairly obvious reasons.

Second, Tuscan had long become the language of the various pre-unitary States' cultural elites, which used it in their works as well as in their correspondence; moreover - poets such as Dante, Petrarca and Boccaccio had an enormous influence over their contemporaries and with Tuscany eventually becoming the birthplace of Humanism, its dialect started being regarded as prestigious. Such a status was already recognised in the late 15th/16th centuries, when the various intellectuals debated on which variety of Tuscan could be considered the purest and suitable for writing!

Alessandro Manzoni published the Betrothed - one of the first books to have a pan-Italian resonance - in 1842, and had a decisive role in steering the debate on the country's official language in favour of the variant of Florentine he had used in his work... something which formed the basis of modern Italian. And in 1859, Piedmont was the first state to made education compulsory (legge Casati), whose pupils were taught in that language.

Of course, Tuscan's dominance was favoured by the fact of it being the most understandable among the dialects.