Okay, fine....so ‘you can’t get there from here’...no wars were ever won that way.
The practical reason Bush did not irradiate Afghanistan is because he would have been brought up on war crimes charges by the Dems at some future point in time, and gotten a life term (until he was pardoned by a sympathetic GOP president). Unless he was committed to living in exile in a country that doesn't extradite to the US (none, especially if Uncle Sam is willing to throw in some sweeteners), his hands were tied.
Bottom line is that the measures we take are limited by what present laws consider acceptable. 800 years ago, he could have sent an expedition to kill everyone they saw, and have had his actions greeted with universal acclaim at the domestic level. Not today.