The Washington Post ran a decent article today on a recent history of US policy toward bin Laden and the Taliban. While it doesn't give a satisfactory answer to your questions, it does make for some good background reading on the subject.
I have some thoughts on some of the past foreign policies in question, on this nation's support of Israel, and on some of the more questionable choices made in the current war, but I'll defer discussion till I can find some cites that I can link to (it's kind of infuriating to not have access to another's source when trying to rebut their argument ;) ).

Bal"Although, I do have some good vague ramblings if you're interested"anthalus