First off, the Hellfire-shooting drones are still test articles, which means there aren't that many available. (The first flight tests were conducted last summer, less than three months before 9/11.)
Second, it's 450 "tanks and vehicles" (emphasis added), not 450 tanks. Probably a lot of the vehicles are stolen trucks.
Third, a tank is a liability without extensive infrastructure to support it. It needs fuel, ammunition, water, new tracks (particularly in a country as nasty as Afghanistan is), and a trained crew. All of these are extremely scarce.