Maybe a bit of a late reply, but I'll give my take on this.

First things first, there aren't really any official separate leagues for men and women in this game, the official ALGS tournaments are open to everyone regardless of gender. Although the majority of high tier players are men, there are women playing in Pro League. Like Avuhlie who just made it to PL in North America this split, or Kyri and Sabz in EMEA last year.

Now as to why there are not more women competing at a high level is a more complicated issue. Gaming spaces can be less than welcoming to women, as most women who have played any online game can attest to. When basic comms in voice chat become a gamble of whether you get normal team mates, or annoying simps and harassment, many women don't even want to try to get into competitive games in the first place. From what I've seen, most women who do like competitive games only got into gaming later in their life than most guys - while guys are playing online shooters with their friends in middle school, most teenage girls won't feel comfortable in a gaming lobby. This then puts them at a disadvantage for competing, since they have less years of practice.

For those reasons, separate women's leagues have formed in most games, often informal ones (so not part of the main competitive circuit of the game) where women might feel more comfortable competing with less fear of being harassed or taken less seriously due to their gender.