I think it's because once you introduce us into the mix (instead of mother nature) then you get mixed up with crap like politics, ideologies, alliances and such. We also get all caught up in worrying about sovereignty and international agreements.
In Rwanda, the rate of killings was higher than at any point of the Holocaust. Yet we in the west did nothing. The U.S., Canada and the rest of the world knew what was going on, they knew how bad it was. Anyone who says we didn't is lying. But no one called it genocide because doing that would have let loose a whole stream of consequences no one wanted to deal with politically.
Ultimately, I think what we all have to realize is that we are all human beings and regardless of a people's race, gender, ideology or nationality, we should be prepared to help if their lives are jeopardized.