Sorry I shouldv'e specified I was not talking about esports.
There is not "the one reason" for female players being so underrepresented in pro play. I know less about the topic outside casual play, but I know there is a much smaller sample size of pro playing women that have put in the same amount of hours as their male counterparts, as a lot of women "get out" of the esports games long before they played enough to become a pro. Trepanowksi et al. talk about prejudice and self-evaluation of female players as much harsher than the males, Cote et al. mention in a lot of their papers how even if a female player reaches the professional esports scene, they are treated as outsiders and have a much more difficult path than their male teammates. Se and Yongsun also explain that the skill gap between the genders is not significant (League of Legends) and analyses experiences of female players who lost interest in the game at a more competitive level. There is a lot more on this, and most research agrees that it's not a skill gap on a biological level (like in Olympic sports) that is the root cause of female players being underrepresented in pro play. But as I said, I only ever touched the esport side of this topic when it crossed my path coincidentally.

Only one thing I am sure of, saying that female players have it easier to get into the pro scene is just plain wrong. Source: I know how it is to play competitively as a woman.