There are states where the ACT or the SAT is required for graduation. That drives a lot of it.
https://www.testive.com/state-sat-act/
I think many kids who aren’t in these states stick with the SAT because they take the PSAT. And if their score is low on the SAT, they look and see if they can do better on the ACT. VA requires neither, my kid took the PSAT, did well, scored a 1540 on the SAT and stopped. There was no need to look at the ACT.