One word: Economics
Sudan is committing the worst genocide of the 21st century, yet no Western nation wants to get involved. It would not be profitable to invade Sudan. They pump very little oil (500,000 barrels a day may seem like a lot, but the country is HUGE). It would take millions of troops to control the country as well. Costs would be tremendous. Other nations in Africa have very little resources. During the Rwanda genocide, no one helped because Rwanda's number 1 resource is coffee. No Western nation is going to stop the genocide to protect coffee beans.
Now I do agree with non-interventionism to a degree. I don't agree with it based on economic reasons. I support non-interventionism based on sovereignty. The US and the Western world has no right to invade a country except for extreme circumstances. If a nation declares war on us, or attacks us, then I believe we should defend ourselves. The US was okay to fight in WWI, and WWII. The current war in Afghanistan is justified as well. Those three wars, to me, are the only justified wars we have been involved in since 1900.
Of course, when it comes to genocide, it is a tricky issue. Does a nation give up all rights to sovereignty when they are slaughtering a whole race, religion, ethnic group, or some other group? Should we invade? Should we do nothing? Should we go there for rescue and protection missions? To me, genocide is a tricky issue for the US. If we invade, the world will condemn us (ex: Somalia, Yugoslavia, Iraq 1991). If we do nothing, the world will condemn us (ex: Rwanda, Darfur). So no matter what the US does, we will be the bad guy in the end. There's so much behind every intervention situation, that it may be years after the event to find out if the action we took was justified.