Fame
Fame isn't the point of getting into this field, but it's definitely an important factor. Without fame, you'll never become a top fighter. And if you're not a top fighter, you'll never become famous. It's quite a quandary.
To get to the top of the food chain and get all those pay-per-view matches and high-dollar endorsements, you've got to claw your way into the spotlight. Many big-time fighters take bit roles in movies to increase their brand recognition. Others make a name for themselves by running their mouths off to the press. As a fighter, you're a product being sold to people.
The best way to get famous in this field appears to be by talking lots of smack, and being able to back it up by laying down ice-cold butt whoopings on fighters with solid reputations of their own. Ever since Ali, people just love a loudmouth. And if you really want to bring on the media frenzy, you can always bite off someone's ear.