I would like to hear a reason why the the ban was left to expire, and why nothing is being done to reinstate it
Because Bush wants the NRA's (National Rifle Association) support. He says he supports the ban, but made no effort to extend it.
Blame it on Congress, for forgetting it all.
Congress didn't forget. They just didn't want to. Bush probably would have left it to run out if it was brought up, though Congress should have at least mentioned it.