It's because the Americans love WWE and loved it during Hulk Hogan's Era and Steve Austin/The Rock's era in the 90s and now want it back but with real, non fake fighting. That's why these trashtalking douchebags like Conor McGregor, Brock Lesnar, Ronda Rousey etc.. are so popular. OneFC is way better. Bellator is also good but they're kinda going the WWE route as well. Rizin is also something to watch.
And Ronda fell for her own hype and acted like a bitch and got knocked out like a bitch. Had she been humble the loss wouldn't have felt so bad.