Boxing (or MMA, or any other individual combat sport) offers a few advantages when it comes to literature. The first is logistical. A boxing match can theoretically happen in any place big enough to fit a ring. Thus they can be, and have been staged in areas from small rooms to gigantic stadiums. Other sports require a specialized field, but officially sanctioned boxing matches have been held everywhere from mansion lawns to prisons.
The second is personal. While there are important trainers/promoters/managers/cut specialists, boxing is a clash between individuals in a way that any team sport is not. The character implications of this are obvious. Finally, the inherent shadiness of boxing makes it a perfect setting for a thriller or mystery story.