Because Rock is predominantly male since the beginning. How many western female bands can you name? I can't name any. I can only name two co-ed bands: Evanescence and Paramore. There has been female-only bands, but they never reached the mainstream.
Even Japan, the land of modern rock, barely has any successful female bands. I can only name SCANDAL. And Ikimonogakari as a co-ed band. While there's been many more successful male bands.