ACT plus writing vs SAT writing sections

Why is it that MIT has said in the past that they don’t consider the SAT writing section in admissions decisions but the ACT writing section is required? What is the difference between the two?

The SAT writing has also been required, but MIT was collecting data to determine whether the writing sections on the ACT and SAT offered them any predictive power about success at MIT.

In the past, neither the SAT writing section nor the ACT writing section has been considered. But these statements have not been made in a few years, and I am not sure whether MIT is now using the scores in those sections in their evaluation.