Ask Lemmygrad

Part of the problem is that the DRC has been unstable for decades (thanks to colonialism), and the area there is huge, hilly/mountainous (especially near the border with Rwanda) and heavily forested, making it really difficult to actually control it against an enemy force. Add to that the many other rebel groups DRC has to contend with, and the distances to cover in order to redeploy troops, they'll be lucky to get back what they lost.

Kivu (the disputed province) is known to be really mineral-rich with gemstones and rare earth minerals, so I can guess that the driver behind the conflict is, yet again, Europeans and Americans trying to secure cheap resources.