Those mountains are INCREDIBLY rugged. While each individual valley is nice and fertile, in many places it's essentially impossible to conquer the next guy over.
Given that the island's been settled for 40k years or so, and that each valley is relatively protected, and that there aren't any adjacent steppes (say) for warrior tribes to come sweeping in from, and the diversity is not at all UNexpected. In fact, it's precisely the sort of place you'd invent if you wanted that kind of diversity.
Compare the Caucasus : even with large neighbours and being near to the cauldron of civilization, AND to the Steppes, you still have dozens of languages, of wildly varying families.