Residing in a place between life and death, the GateKeeper keeps notes of the names of those who have died. At the moment, Hell is full so he must send anyone who dies back. If they do not have a way to be put back into their body correctly, these souls become Hungry Ghosts. If the Gatekeeper were to squeeze anymore souls into Hell, the excess would be pushed into Oblivion.
This is not to say that the Gatekeeper does not worry about the problem. There is currently a quest being led to find a way to solve this problem.