I'm not entirely sure, it's probably a combination of several factors. Most of this is just speculation, so don't take it too seriously - these are just my possible ideas:
-Women see that tournaments are almost entirely dominated by men, and that discourages them from competing in future tournaments, especially if they experienced any sexism (I don't know how often that happens, but it almost certainly does happen to some people)
-In more traditional cultures women are expected to be housewives and will not get as many opportunities to seriously train and become better at chess.
-Women are encouraged to pursue women's titles and championships (like being a WGM or being the best woman in a particular region) rather than being encouraged to pursue the more general titles.
I remember Hou Yifan talked a bit about the difference between men's and women's chess, here's the link if that interests you https://www.chess.com/article/view/hou-yifan-interview-chess
The top 80 world ratings are all men but how is this possible? I know women have a disadvantage in physical sports because of biological differences but chess is a mental sport