I don't think the language thing covers it.

Plenty of European countries have more than one language, only recently got 1 unified language.

You have countries like India that manage to make a cohesive country out of disparate peoples.

That's not to say these aren't contributing factors.

My current 'theory' / observation is that sub Saharan Africa never really had large scale civilisations, I wonder if that colours their conception of what a nation state is. Eg, if you think in terms of tribes. Perhaps that hinders scaling up to something bigger.