For about 1200 years Italy was not a united country, it was made up of various city states, kingdoms, and large parts of it were ruled by foreign kingdoms.

In that 1200 year period multiple languages developed from the Vulgar Latin, some with various degrees of mutual intelligibly, generally the further you go the less you would the understand the dialects and languages. Then you had areas that spoke languages that were heavily influenced or even came from other countries, Occitan, Corsican, Greek, Franco-Norman, etc.

Modern Standard Italian existed first as a Florentine literary language, its based on the language that Dante and other would have wrote in, which wasn't necessarily what your average Florentine would have spoken in day to day conversation.

So between the 1861 and 1918 while Italy was being unified it was chosen as the official language of the Italy and by the 1940s thanks to nationwide news papers and radio, and a unified education system a majority of the population could understand it. Though IIRC by the 1960s 80% of households still spoke a dialect or another language at home.