I think one reason it hasn’t changed to use the SAT is because of the test prep, etc, that goes on with the SAT.
The PSAT is an early snapshot. We see kids who did “ok” but not well on the PSAT start “studying for the test” for the ACT and SAT, and suddenly they’re top 1-2%.
It’s like those TIP programs that identify smart 5th graders using standardized tests and then have them take the SAT or ACT as 7th graders. I bet that many NMFs were also TIP kids.