Sometimes it could be a case of a rare combination. I don't know if this is, if you have access to pop reports you can check. I know pop reports don't have every known example out there listed, but it's an ok way to gauge the scarcity (in a way).
This card is graded "poor", but a friend checked the pop reports for me and it's the only one graded by PSA with none graded by SGC. So, I figure it's a rare combination. That doesn't mean someone else doesn't have one- and I know PSA didn't used to keep track of the backs....but, the fact that SGC doesn't show one is a good sign of a tough combo.
If this isn't the case, I don't have the answer
Sub set collector who hasn't seen one forever? Don't know. It's a nice card though.......
Sincerely, Clayton