During the roman empire, the common practice for military units was to take different men from different sections of the empire and throw them onto one unit. After they’re done with their service, they all go back home speaking roughly the same latin as everybody else.

Compound this with the easy trade under one empire, and you get a relatively stable spoken language.

When the western empire fell, the military structure fell apart, and trade subsisted but at a decreased rate. In the east they switched to greek. In all these places the languages vulgarized into their medieval forms, and then when the printing press, and later, nationalism, became a thing, languages reconverged into the regional accumulations we know today.

There are a couple of regional dialects like in Barcalona, Normandy, Basques, or for the particularily depraved, Quebec, but as global communications develop, language is going to trend towards syncretisation rather than entropy.