The big difference is that the whole situation in and around Rwanda was just a modest border observation force. It was the surprise that caught the UN out, because all of a sudden the president's plane was shot down and all hell broke loose. That's when the genocide started. Before that, it was a very modest observation mission on the east boundary.
Having been surprised, the UN didn't want to augment the force. They did, modestly. Some idiot had put a previous colonial contingent, the Belgians, on the ground in an area where the Belgians weren't terribly popular as one side in the conflict. It was a ridiculous UN decision to put the Belgians there, and it put them in a very difficult position. Subsequently, after the slaughter, they went home.
It was the surprise of Rwanda, both on General Dallaire and on his force. That wasn't what they were supposed to be dealing with. They were supposed to have a simple border monitoring mission, much like a conventional peacekeeping mission. It didn't turn out that way.