This reason kind of ties in with the last one. In 2011, if someone is interested in watching the highest form of legal pugilism, they don't watch boxing; they watch the UFC.
The emergence of mixed martial arts and particularly its most elite level, the UFC, has served as a reminder of what a combat sport has the potential to be. UFC matches are constantly action packed; boxing matches make you feel like you won the lottery if you find a fight with consistent aggression from both fighters.
At some point, boxing became about winning the match instead of beating your opponent. Boxers almost seem as though they're fighting for the approval of the judges, rather than to dominate the other fighter. If you watch just about any UFC fight for more than 30 seconds, it is abundantly clear they're fighting to beat the other guy in the ring, not to beat the guys keeping the scorecards.
Furthermore, UFC almost always gives fans the most elite matches possible without any of the dancing around and game-playing involved with trying to nail down a boxing match. At the end of the day, the UFC just gives boxing fans more of what they want to see than boxing does.