Because I think Americans are usually only interested in intervention when the situation seems to call for war -- when it can be painted as "defending the country from tyranny."  If it's not seen as an absolute necessity, then they don't typically want to be involved.  (Note the emphasis on "seems" and "seen".)  That's not always the case, and I submit to you that, were we given proper media coverage of Darfur, the citizenry would want to intervene.  But I think it holds in general.

That's, obviously, just my opinion, based on my experience and reading of the country right now.

Otherwise, they tend to see it as simply putting soldiers in harm's way for no reason.  Remember that George W. Bush "won" in 2000 partly on his insistence that the US not be "The World's Police".  Obviously he changed his tune when becoming the world's police was seen as a political opportunity, thanks to a public that was scared out of its collective mind and wanted to bomb the hell out of something in response.

Be nice to America. Or we'll bring democracy to your country.