They're not "banning" them... they're simply dropping them. For several reasons:
1) They're expensive to run. The school has to pay for all of the equipment, maintenance of the equipment, replacement of equipment that is damaged beyond repair or stolen, and supplies for the projects.
2) They're a liability. Teenagers and power tools are a lawsuit waiting to happen.
3) It's getting more and more difficult to find someone who actually wants to teach those classes, because the salary is low, the class is underfunded, and it's usually full of students who just want an easy A with the least amount of work possible.