> Why did it take so long for something so obvious to be acted upon?

A few reasons:

1. Sex is not as straightforward as most people think, and what to do with intersex people is not clear.

2. Trans athletes are underrepresented at pretty much all levels of sport, and aren't actually winning that much, making it not actually an urgent problem.

3. The philosophical underpinnings that advantages due to differences in body development should be disqualifying is a little broken, since we do not consider Michael Phelps being double jointed as being an unfair developmental advantage.