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The SAT is in no way more "rigorous" than the ACT. The theoretical difference between ACT and SAT is a slight shift between "achievement" (ACT) vs "potential" (SAT) although the reality is that this is pretty minimal. That is to say, ACT was supposed to slightly more reflect what you had learned in HS, while SAT was slightly closer to something like an IQ test that changed less by HS education.

In practice, the only difference between ACT and SAT for decades has been geographic preference. East&West coast schools primarily use SAT and Midwest/Central schools primarily use ACT.

The relationship between the ACT and SAT may be changing as the latter is watered down. Or at least MIT has found as of late the ACT is a better test for its purposes.

It used to be that in the Midwest only a few people took the SAT, people looking to get into schools that required/preferred it or people looking to get a scholarship that used the score. The state schools were all fine with the ACT.

I think the reasoning is that we should allow less able students more time to understand the material, there isn't much point teaching calculus if someone struggles with basic algebra.