Kenny has a healing factor[]
How is it that Kenny keeps getting killed in one episode, and comes back the very next one? How is that even possible? If he had nine lives, he'd be dead by now. So this points to one other answer. Kenny is a mutant. What's his mutation? He has a healing factor.
Jossed: During the "Coon and Friends" story arc it is revealed Kenny's immortality can be traced back to the Cult of Cthulhu.
Eric Cartman, Stan Marsh, and Kyle Broflovski break the 4th Wall[]
You know how Cartman, Stan, and Kyle always say, "Oh my God. They killed Kenny."? Whenever he is killed by animals, aliens, and humans, this quote makes sense. But what about when he is killed by inanimate objects? They still use 'They' and not 'It'. What am I implying? That Cartman, Stan, and Kyle somehow break the 4th Wall.