Why do some people get so charged about Darfur?

This is something that absolutely puzzles me. There are so many problems in America. However, some people still insist on sending money to third world countries that America doesn't even need. If that's how the left wants to look at the world, then you will have to rescue all of Africa, 1/2 of Latin America the Arabic states, and the nations of Indochina. The point that I'm trying to make is that America cannot be responsible for atrocities that are happening in other parts of the world. America has to pay off its debt (which is absolutely huge), invest in energy independence, spend more money on education, repairing outdated (and potentially unsafe) infrastructure, etc. That being said, why do some people still insist on making Darfur one of America's priorities?