I don't know anything for a fact, but if I had to guess?
1. The French didn't really care to take the time to look at what would be best for the West African peoples. Just as most colonial governments didn't really do what was best or most optimal for the territories they colonized. They just left when it became obvious they'd have to.
2. They probably thought, to some extent, they'd be able to play these groups off against each other, post-colonialism. If you create a stable country of culturally and linguistically homogenous people, odds are they'll be more productive (in an economic sense, anyways). But if you create a bunch of unstable countries, without regard to (or perhaps because of) ethnic/cultural tensions and differences, you can exploit them longer.