Men's sports are simply exponentially more popular than women's sports. I think that fact can be traced back to gender expectations, some of which are socially constructed and some of which are biological.
Some would argue that men's sports are more popular for the simple reason that men are bigger, faster, and stronger. That might be true, but I think physical traits that are distinct to gender often say what we want them to say, and because of that I believe the popularity of men's sports derives primarily from cultural roots. Every young boy is exposed to an environment that views athletics as a celebration of masculinity. But to the extent that men's sports are celebrated while female sports are minimized is also somewhat arbitrary. They are different, yet different in ways that are not necessarily inherently better or worse (obviously, a men's team would beat a women's team on the scoreboard, but that doesn't make it a better product; many people on this board prefer college basketball to the pros despite the fact that the quality of play is vastly inferior).
I'll be interested on the direction you take your project in. What I've always wondered - and continue to wonder - is whether female sports would surpass men's sports in popularity if the platforms were reversed. What if instead of people like me growing up watching men sports, we grew up watching female sports? That seems preposterous and maybe it is, but the way we would react to that hypothetical would likely denote whether there is an intrinsic appeal to men's sports or if it's an extension of culture. I'm not sure if that makes sense to anybody besides me, but if we were told - explicitly or otherwise - from a young age that female sports were superior, I think there is the possibility that we begin to conceptualize female sports in a completely different way.