If you watch 12 rounds of boxing, especially if it's a boring one, you'd change the channel. If you watch MMA that is about 3 rounds, there's that quick anticipation, and more interest since it does not last long. I think fighters tend to fight harder in MMA due to its quick rounds/durations. So that's why there's more of a fight in MMA than a 12 rounder boxing match (if it's boring). If boring fighters like Hopkins or Winky or Floyd realizes that they only have 3 rounds to fight, they'd fight more and in the end might get caught, thus making it an actual fight, not a sparring session. Much like a hit song is usually between 3-4 minutes. No one can sit listening to a 10 minute song, well not many people anyway. Like most movies are under 2 hours, usually 90 minutes. Who can sit in a 4 hour movie? Very rare. Much like a TV series is between 30-60 minutes.