I think niche is being used in a couple of ways in this thread.
In terms of the professional entertainment industry, women's soccer is still a niche sport.
In terms of participation, girl's soccer is main stream.
And in terms of overall money, including selling shoes and other equipment, kids sport is very big money, much bigger in fact than the sports entertainment industry -- which is why there are so many sports equipment manufacturers advertising on sporting events.
Kids sporting gear is an especially lucrative market, because parents will generally spend far more on sports activities or their kids than sports viewing for themselves, and kids tend to outgrow equipment quickly.
Which is why ESPN is happy to give a lot of time to the women's WC -- advertisers like to see something that a lot of kids are interested in.
Overall (including participation and entertainment) girls soccer (though not women's soccer) is much bigger financially and in numbers than MMA. MMA is never going to be a sport that many parents will want their school kids doing, so its never going to have access to the same money.