I think that the "obsession" with Boxing/Combat sports originates from the amalgamation of many things:
1. The concept of absurdity, in which we all see these happy, vanilla girls, who are usually just cute and dandy, in combat gear ready to fist fight until blood spills. That, on itself, it's funny and appealing on it's absurdity alone.
2. The feeling of dominance; Such sports inspire dominance, the attitude of an alpha individual. Those with submissive fetishes would enjoy being under such powerhouses.
3. Femme fatales.
4. Other misc fetishes like blood, pain, sweat, hitting, choking and the likes come true. It's also a Vanilla scenario where you can enjoy, guiltless, of the pain of your Waifu while getting hard without needing to feel like you have questionable morals. "It's an entertaining sport, after all".
I see the appeal, having practiced the sport myself and loving it. This applies to both the female and male gender. Also, the aesthetic of combat sports is also fun, interesting, and to a certain extent sexual. Like seeing those manly gladiators fight to death, expressing qualities of power and manliness. The gear and visual appeal is also interesting, and the presence in modern media as a top sport is also important.