[glynparson]

Mainly because of set registry competiton. There are far more people working on Registry sets with PSA than SGC. This card fits into the 1963 set as well as the Pete Rose sets all of which have heavy competition. Also some hall of fame rookie registry collectors might be in the competiton figuring he may someday enter the hall, probably posthumously.
I am pretty sure this card was probably attempted to be crossed to PSA at some point and probably rejected. That doesn't mean it isn't nice enough to be a PSA 10 they just probably didn't want to make that call in the holder. Even though I understand most of the nuances of grading I don't understand the price differences between 9 and 10. I get the premiums up to most levels but the difference between a 9 and 10 is so often just grader preference that I really don't get the prices paid for Gem Mint. You can usually see a difference between most grades, if you truly study the cards, but 9 and 10 the differences in my opinion often don't exist.