The cult of JRE fans helped give its popularity another boost, but in reality, what other league are you going to watch save a different fight sport like boxing? People like to huddle around the cooler and focus on one league (See: NFL, MLB, NBA). That's not to say WNBA or XFL don't exist, but think about the multiplier of games you'd have to keep track of with your friends for all of them.
Natural entertainment monopolies...interesting concept.