I imagine because being nationally standardized, the SAT/ACT more reliably correlate to IQ. A 4.0 GPA means nothing but an A in all classes, which may give some

I imagine because being nationally standardized, the SAT/ACT more reliably correlate to IQ. A 4.0 GPA means nothing but an A in all classes, which may give some insight into Conscientiousness, but it says nothing about what the classes covered, what the school averages at, and is thus harder to correlate with IQ. Since IQ+Conscientiousness is better than either alone, of course they'll want to consider both.