I would imagine that there would have to be a really good reason to happen, and the default is millions of different (albeit slightly) languages amongst an equal number of small communities. It takes empires and states to force a unified linguistic project, which is not necessarily pursued in all cases. If you've ever had a group of friends sort of develop their own cant, imagine how quickly it could change if it was 150 people who only contacted outside traders five times a year.

Language and politics is a huge part of linguistics (e.g. "a language is a dialect with an army and navy"). Certainly, since nationalism began there has been concerted efforts to unify languages around the powerful members of a nation (France explicitly does this with a legal structure, English has elitism in social structures). The borders of languages are forced categories of fuzzy culturally evolved systems. Who decides the line between German and Frisian?

The short answer is "Why would there be such a broad language?". The default case is diversification, being able to talk to someone across the world might be convenient every now and again compared to being able to talk to your local community every day.