Many MMA fans began as boxing fans who are now totally and completely disillusioned with boxing's corruption, top fighters avoiding each other, insane amounts of belts and weight divisions, etc etc etc.
I know that I started as a boxing fan and switched to MMA because the state of boxing is totally fucked. I like combat sports in general, and the reality is that at this moment and for the forseeable future, MMA is where the action is. Simple as that.