Catfights are a fantasy; the idea of two beautiful women, with a grudge, being so consumed with jealousy, rage, and emotion that they are willing to act out of character, and fight in a fantasy way that includes stripping each other, puling hair, entwining legs, and grinding away body to body as they roll on the ground is just....hot. Most catfight fantasies are by design unrealistic; they're not fulfilled (to me) by watching two tatted, drunk skanks trying to hurt each other in a parking lot behind some dive bar. Trish from California Wildcats had it right; it takes a certain fantasy scenario, and a buildup, and two beautiful evenly-matched women to make a catfight "hot"
Re MMA, two women who train professionally, then enter a ring to fight like men and pound the snot out of each other for a cash prize just doesn't have the same attraction. They're not "women" in a fantasy sense anymore, they are professional athletes engaged in a sport. It lacks the emotional element, especially when the women are not particularly physically attractive by my perspective (the lack of make up, cornrowed hair, tattoos, the mouthpieces, and voluminous baggy shorts just don't appeal to me) except in some very rare cases (Paige Van Sant is hot).
No question MMA is more "real", and the participants are serious, professional, well trained athletes. But I don't find it "sexy".