Small brains, thick skulls, and low metabolisms. Kinda like reptiles. Also, they play dead.
Please go and kill it properly. Crush the brain, stop the heart, or both. Preferably decapitate it to be sure. Even pest animals deserve a clean death. Possums have a very low rabies risk and aren't the fastest animals, you have absolutely no reason not to go out and put it out of its misery.