They have different grading standards. It would be a mess. Some people would be really disappointed with some lower-graded crossovers.
I like SGC, but grades of 2 and below get really generous compared to many PSA examples. You can have holes and markings on a card and still get a 2 with SGC. There's other differences up/down the number scale between PSA/SGC, but it's really evident in 2 and below.
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