Fighting in general is a boy's game and so are a lot of other sports. Girls are generally not as encouraged to do these sports as much as boys are, so the quality isn't going to be as high.
In general fighting is associated with boys, yes, but ask for example Israeli women who do their duty in the military, or look up history where for example Vietnamese fighting women were essential in stopping Chinese occupation in the early days of Vietnam. There probably won't be as many MMA women as men in the near future, but nonetheless as the business grows also the talent pool will rise. And quality-wise women MMA is not really worse than HW men MMA where apart from some exeptions some blown up muscle guys just brawl without any technique worth mentioning until one of them gets a lucky punch or runs out of gas after 3 minutes.