There were actually a few reasons.

First and foremost, the Rwandan case happened at virtually the same time as Somalia. It drew an uncomfortable parallel for the United States in particular, where U.S. casualties had created such an uproar that Clinton withdrew. The genocide in Rwanda began just a month later, all but guaranteeing that there would be no U.S. will to intervene.

Another reason was the deliberate attack on Belgian peacekeepers by the militias. They kidnapped, separated, and brutally mutilated ten peacekeepers if memory serves, making an example that caused an outcry in Belgium (who had the best troops in the peacekeeping force in Rwanda's UN contingent) that the troops of Belgium (and many others) were withdrawn before they too could be killed in that way. Calculated brutality scared away others, in that sense.

Another reason was simply a lack of expertise motivated to take action. Not only was Rwanda an obscure country hardly on the State Department's radar, the Pentagon seemed loathe to take action in coordination with experts on Africa because it was hypersensitive to casualties and costs after Somalia.

And the last reason often given is that no one had any stake in the issue. The world has historically avoided taking action in places that it does not have a stake in, and Somalia made that thought process feel justified. If you compare the response of Rwanda to that of Yugoslavia, it becomes clearer; the U.S. and Europe had more of a stake in the consequences of the Yugoslav crisis, and though reluctant, eventually carried out airstrikes on Serbian forces that were carrying out ethnic cleansing. Whether they were effective or not is one thing, but the intent was there, unlike in Rwanda.

One last thing; the Rwanda case happened quickly. A large amount of the deaths occurred in the first three weeks, and such a short timeframe to organize a sufficient response of the necessary scale would've made it difficult. I don't believe this was a factor necessarily because there could have been other ways to help, even if not altogether halt the genocide, that were not taken.

I should also add that another common accusation is racism. The argument is essentially that Africans matter less than Europeans, for example, so Rwanda got no response while Yugoslavia did. This may have some root, but there's no denying the geopolitical and historical factors at play as well, so accurately measuring the extent of each factor's influence is difficult.

A fantastic read on this is by Samantha Power, called "Bystanders to Genocide". It's an article, but a long one. I highly recommend it. It's U.S.-centric, but it does a good job of laying it out.