What happened is that the Taliban turned Opium into a cash crop. It became more lucritive for the farmers to grow it than food, so they grew dope. That's one of the reasons that their asses are starving right now. The Taliban also taxed the crap out of the growing, transport, processing, etc. which is where they were getting much of their funds. As long as it was going export, they didn't care.
There's not real good way to destroy the crops with bombs. Spraying is not fesible from thousands of miles away. Especially when that would be one perfectly good plane that could be dropping bombs instead. Secondary reasons are that you may not want to be spraying after the Anthrax attack. Many may see it as a possible NBC retaliation, since the Soviets made widespread use of chemicals there. All it would take is one clown in a field to get sick or die, and we'd be the bad guys for using "chemical weapons".
Lastly, we aren't there to install new governments, straighten out the Afgahni cultrue, restructure their economy or fight drugs. We're there to get bad guys. We probably have some responsibility to get some things straightened out there, but I don't think we're too interested in taking on a whole lot of perifieral missions.
Ross