I think CaptionRon said it better than I could, basically Tennis is a whole other ball game to combat sports. As I said earlier, sports involving grace and form are much more easily accepted and interesting in the mass public as women's sports. Blood and brute force, not so much. It just doesn't have any appeal. Frankly, for combat it looks weak as hell. The performance difference is far greater than sports like tennis / badminton / gymnastics etc. because of the greater importance of brute force, power and pure athleticism.
Women's boxing would be a better comparison, and just see how that's doing even after being around for decades.
Also I don't see Dana doing it as a cash out. Ronda is no bigger a star than the biggest women boxer, the casual fan would pay 10 times more for the worst "GSP beating up cans from the bush leagues" fight than they would for the biggest female mma fight in all of history. People just don't care, apart from a small niche fan base. It doesn't matter if Cyborg is pumped up with enough steroids to be a male horse, she would still get her ass whupped handily by mighty mouse and people had trouble getting interested for that flyweight title fight.
I see it more as Dana getting some, hence his sudden flip flop on Women's MMA. Right after him and Rousey being seen together alone at many red carpet events. You don't see Dana attending movie premieres alone with Jones now do you? Certainly haven't seen him sporting nude Anderson Silva T Shirts.
The profitability of this fight or the division don't matter to Dana, this is just dole for him. He's rich enough to afford a few indulgences.