The simple answer is because men have bodies and angels do not. Basically, man is an embodied spirit. His spirit is “within” (for lack of a better word) a body that is subject to time and change. Because he is capable of changing, man can repent and turn back to God. Angels, on the other hand, are pure spirit and have no body. Because they are not subject to time and change, they cannot change. When the fallen angels made their choice against God, it was a permanent choice because they cannot change.
