It would be nice to give Jim Henson some credit, but there are legal issues with "created by." In television, a "created by" credit goes to the writer(s) credited on the pilot episode. Things would get a little murky since some characters were pre-Muppet Show and some were post-Muppet Show, but since they're largely from that series, Jack Burns would probably be included in a "created by" credit. I would guess the lack of credit comes down to a host of legal issues, not the least of which is the fact that there's a company that bears his name. Still, you're right; it would be nice to find a way to give some recognition to Jim Henson.