Same reason they have scholarships and placements aimed just at women or just at ethnic minorities in other areas in which they are underrepresented, like science. It's to provide opportunities and raise the profile of Women's Chess. This is helped by preserving the Women's comps just for women, though many comps are Open/Mixed.
Didn't you go on a course that was exclusively for a particular minority group of would be comedians? The reason for it being just for that demographic wasn't because they aren't as funny as everyone else and can't compete, was it? It was to provide opportunities and a safe space. Perfectly good reasons.