There has been a lot of discussion, writing, and opinions over the years as to why the majority of top chess players are men. There are some notable exceptions, of course, but the disparity remains.
My own observation is that reaching the top levels of chess takes more than simply intelligence, but requires hours and hours of study and practice in addition. Some say as much as 10,000 hours of study and practice to reach grandmaster level, much of it during the formative pre-age 20 years. Girls for some reason tend to lose interest in the game about then, at least from what I have seen.