Well, since they harbored an international terrorist that led the group that attacked us, we went in. Funny this scumbag can't remember that much.
As to why we're still there, I dunno. Liberal, politically correct wars last longer I suppose. You know, the ones where you tie your soldiers' hands behind their back, make them build schools instead of killing enemies, etc. Seems to me it should have been over about 2 months after they refused to hand over Osama and we carpet bombed them back to the pre-stone age.
As to why Obama has us still there is a further mystery, given his penchant for mooslims.