It was easier for Belgium to end slavery in the Congo, as they were the ones who started the slavery in the Congo. The US is in a bit of a different position. Rwandan warlords are not nice people, and are generally tolerated by a Rwandan government which is itself corrupt. They want to go over the border and kill, maim, steal minerals, and enslave, then what can you do if you don't want to risk direct confrontation with them?

This trade is difficult to break in the current environment. We'd need to get tough on people who we have generally not wanted to get tough with because we want to keep them friendly for a lot of reasons. Not least of which is growing Chinese (and European) influence in the nations of coastal Africa. We know very well that some of the people we need are less than savory. Some of the trade we engage in is less than honest. But it's not just the coltan trade that's influencing our behavior here. There are a lot of different and competing strategic considerations at stake.

My own opinion? This is probably going to blow up in our face in the future, and we'll spend the latter half of this century, (or maybe the first half of the next?), attempting to convince a rapidly developing sub-saharan Africa that we're deeply sorry for the past but you can trust us going forward.