It has to do with the historical amount of injuries that happen on K/O returns. Typically you don't put guys back there that don't have a huge part in your offense. That's a big reason why the moved the kicker up 5 yards and now most of them go out of the end zone.
But with that said, I do agree with you. He should be back there. He may be on the smaller side, but he's built pretty well for his size. Also, he's turned out not to be a big part of the offense with Shittyhiney at the helm.