That's the problem, how do you make sure there's no cheating?
Athletes are being tested for signs or taking extra test all the time. But how do you decide what the minimum estrogen level is? Or otherwise, how else do you prove the trans woman is consistently weakened enough for it to be a fair challenge?
I do agree it should ideally be up to the leagues and not a national level political decision, but there are so many different sports, that's a lot of people that have to worry about the same problem.
So again, be easier to just let them have their own leagues. They wouldn't fare very well in the unrestricted gender leagues (known as men's leagues for a good reason) either.