Effects of colonialism on Africa's past and present

That has nothing to do with it.

When you create a literary language, you take one dialect and use that. Eventually some things from other dialects will sip into the literary language. It's been done in Africa and even in Europe with languages like Norwegian. And native languages in the U.S. In fact even literary languages like French were formed in this way.

There are mutually intelligible languages with completely different names, as there are in Europe as well, but the main problem with Africa is that the African languages are more often spoken than read/written, and are seen by some as inferior to colonial languages like French, English or Portuguese.