I think the reason is: at the same skill level, it's more difficult for someone to succeed competitively as a female player.
When a female player starts playing the game, every day they receive more harassment than other players exclusively because of their voice, even at radiant level.
When a female player thinks about competing seriously, they're less likely to make that decision: There's less representation for serious females competitors (before C9 White in Valorant).
There's more risk looking for teams: What if I'm harassed? Can I feel safe in a team environment, when it's very likely that I'm playing with four boys? Do I need to live in team house physically? Will I feel welcomed? Will the team/org not pick me up because they don't want to deal with these issues? Will my skills be not acknowledged because I'm a female talent?
Of course, the female players with skills will shine no matter what. But every story of success comes from long time of practice and looking for opportunities, and there are so many more reasons to give up. The ones we see in main scene today pulled through from disproportional difficulties than their male peers. The male players rarely have to think about these things. But these little moments, compounded together, make up an experience that marginalize female players, and encourage fewer of them to stay in the scene.
If we recognize the disproportional difficulty for female players to compete, it becomes obvious that co-ed teams won't happen overnight. Fair competition is not a reality right now. Then spotlighting on marginalized groups and offering them opportunities to enter the main scene is a temporary but great solution, until a point when we can call it a fair competition.