As far as pay goes: the men's league brings in about 40 times more revenue, hence more pay.

As far as competition goes: when you get to that level of competition, the absolute cream of the cream of the cream of the crop, men have a very observable natural advantage due to the physical advantages afforded by testosterone and male puberty.

This to to say, while many women are bigger, faster, and stronger than me (a fairly average male in those aspects), the biggest, fastest, and strongest have always been male.

I have read some things about average male response times and hand-eye-coordination being naturally better as well, but that may be less documented and observable.

Basically when you are selecting the one-in-a-million for athletic performance, men have a natural advantage.