Because the aura is an "enchant creature YOU control". Since you do not take control of the spell, only changing its target, the "you" is still refering to your oppponent. So you can at best redirect the aura onto some other creature under your oponent's control, if there is one. You cannot redirect it to your creatures, since those are illegal targets.

If you try to redirect the trigger, you run into the same problem. The target is a "nonland permanent under your opponent's control". Which again refers to the opponent of the trigger's controller, which is you. So at best you can redirect the trigger to some other nonland permanent of yours, but you cannot redirect it to something under your opponent's control.