Facepalm

Because there's a cash prize involved. Many male runners could easily snag the prize that's supposed to encourage women to also participate in sport.

Yes, you can find plenty logical flaws in this system. But it's a matter of perfect being the enemy of good.

The hope is that it's better for gender equality to also offer prize money for the fastest of some rather arbitrary group ("women"), because then they have any chance of winning at all.
The downside is that it leads to all kinds of conflicts, because what in the absolute flying fuck is a woman?