I suspect this is just getting rid of redundancy. Everyone taking the SAT subject tests is probably taking the equivalent Advanced Placement test anyhow.
However, the biggest problems with these tests (and the AP tests) is that they are expensive--they cost a non-trivial amount of money to sign up for and they cost a lot of time to prepare for if your school doesn't offer direct AP classes.
However, the biggest problems with these tests (and the AP tests) is that they are expensive--they cost a non-trivial amount of money to sign up for and they cost a lot of time to prepare for if your school doesn't offer direct AP classes.
The expensiveness is the barrier.