It doesn't need to have anything to do with race. Things happen for incredibly complex reasons.

Overall, bad people try to seize power -- that's a constant the world over since bad people will always exist and they have less moral inhibitions than anyone else. It takes a lot of good luck, courage, and a tremendous amount of organization[1] for good people (or more commonly, people who are at least not outright bad) to prevent it -- and even more of those to reverse it once it's happened. And the struggle will never end.

Bad and incompetent people mismanage everything since their only priority is self-enrichment and power, so because they won most of the struggles in the poor countries, very little has improved for the common man compared to what should have been possible.

The only way that things will improve in those countries will probably involve tremendous bloodshed - revolutions. The leaders who can't or purposely won't let commoners share in what wealth is available aren't going to just spontaneously abdicate.

I want to point out that I actually don't blame Ugandans or any other poor-country commoners for not overthrowing their corrupt overlords. If and when they do, it will require a tremendous sacrifice of lives to achieve it. I know I would personally not have the cojones to charge the palace armed with rocks and clubs -- whether I had a crowd of 10,000 other freedom fighters behind me or not. I'd just deal with the banal bullshit of the regime and make the best of it to avoid the high likelihood of getting shot.

[1] by this I mean the organization of a good government system -- a constitution for example and empowered courts and law enforcement to actually enforce it. Or in its absence, organization among incredibly-brave individuals to build their own system that can outmaneuver the bad leaders.