I guess would be that boxing games don't sell enough to justify getting into the market for other publishers. In more business terms, there's a big barrier to entry to create a game in that space: You have an established franchise from the biggest player in the market - EA, this makes it unattractive to compete. Plus you have the expense involved in finding or establishing a development team to research the sport, develop the engine, create the content, etc. Lastly, you have potential licensing fees, since in order to compete at all with EA, you'd need some sort of official sanction from the sport. Add all that together, plus the fact that you'd only sell a small number of copies, and you have a high enough barrier to entry that will deter just about everyone else out there.
All that said, Legendo Entertainment has created a boxing game for the PC called Heavyweight Thunder you may want to check out. 