It might be it's "boring", by boring I mean women don't jump that high and look at WNBA game and then at NBA; women play less rough (even though atacks are agressive), more delicate game than men, even though woman players aren't less talented and skilled in WNBA than men in NBA. It's not only in USA that women's basketball is leass popular than men's, but in other countries too. For example in my country men's basketball is number one sport while women's bsakteball is dying out in men's basketball shadow.
1- NBA has been around longer 2- the NBA and networks fucked up the marketing for the wnba 3- lack of skill... Relatively 4- no cool dunks 5- less marketable players and teams. Especially from the college level 6- most of the girls are butch as fuck
Because it's existed longer and is fully established with its own stars and junk. Why would you watch another version when you could watch the one with the people you already like? It's like choosing between Dark Souls or Lord of the Fallen.