There's a theory in linguistics that languages tend to complexify if they're used by a small, insular group, but tend to simplify if they're used as a trade languages. Evidence for this theory can be found in languages like English and Swahili, which are grammatically simpler than their Germanic and Bantu cousin languages. You can also see in Caucasian languages that the languages spoken by higher altitude speakers (who were more protected from the various waves of conquest in the area) being more complex than the lower altitude speakers.