If you have to ask this particular question, than you wouldn't care what the answer is any way.

In most states it is illegal to shoot a deer from a "wheeled" vehicle, as the law states here in Wis.

Second, it removes the "fair chase" aspect of hunting. The deer trust the tractor to be non-harmful. They trust it.

Third, its unethical. But since your asking why not, your not going to give any credence to this answer so it doesn't matter.