I used to work at a place where people dressed relatively formally, but there was no official dress code. There were some interesting things about the way different people, particularly men, dressed. It was acceptable for more junior people to dress down slightly- you could get away with not wearing a jacket, and also with occaisonally not wearing a tie. It was never acceptable for higher-ups to do that- people above a certain level were never seen without ties, and almost never seen without jackets either. A suit is thus a marker of status- anyone above a certain level needs to be wearing one. Note that it's not that people are just choosing to do this, as any senior person who didn't wear a suit would be violating the norm and hence look the ridiculous- that's just how the norm takes shape.
The social norm about suits in most places in Africa that I've been is that Africans in any kind of high status job (say, any job that would require a university degree) always wear either suits or (sometimes traditional clothes). If you're a foreigner, though, the norm is to dress very informally. Why might this be? Well, particularly if you're a foreigner, it's very hot and uncomfortable to wear a suit. And there's already a rather obvious marker of your status- i.e., you're white- so the suit doesn't convey any extra information about you. Again, it's not that people make a conscious decision based on these factors; these are just some incentives that might explain why the norm evolved that way.
There are two instances I can think of where I've seen a much more flexible norm about wearing suits- one is in Burkina Faso, where I am now. For whatever reason, the norm is that even very important Burkinabe are often dressed casually. Yet, some of the locals wear suits anyway, even though it's very hot (it was about 103 this afternoon). My guess would be, the suit-wearing norm evolves in the kinds of jobs where you are both operating from an insecure position, and you have to impress foreigners (like sales, or trying attract foreign investors). The suit wearing-norm may have been subverted among the Burkinabe somehow, but wearing a suit is still a way to signal to foreigners about your status.
The other situation with flexible suit norm is among coaches of European soccer teams. Some wear track suits, while others are dressed to the nines. I have no idea what it says about if you're a suit-wearing soccer coach.