Years ago there were many colleges that required subject tests with the SAT but accepted ACT in lieu of submitting SAT and subject tests. The why had two factors. The ACT was considered somewhat more of a test that tested information you actually learned in high school while the SAT was considered somewhat more of a test of your general knowledge. The other reason was that some colleges considered cost in making the decision. You could always send to colleges both the SAT and subject tests that you had taken in one order while sending an ACT and subject tests doubled your cost.
That divided rule today actually no longer exists. That is because most schools that required subject tests in the past have either stopped requiring them at all or changed the requirement to a recommendation. There are only four US colleges left that actually require subject tests – MIT, Cornel, Harvey Mudd, and Caltech – and those require them regardless of whether you submit SAT or ACT. There is only one college left that requires subject tests with the SAT but accepts ACT in lieu of both but that college, McGill, is from Canada not the US. Even Duke now recommends rather than requires subject tests. It appears to be the only US college that still makes a statement of difference in that tests are recommended if you submit SAT rather than ACT. Others that have changed to recommending subject tests now mostly do so regardless of whether you submit SAT or ACT.