I believe its simply because boxers take all the hits to the head well 70% the rest to the body. MMA fighters can win in several ways and the fights are so random you cant predict anything. Boxing you can for certain watch a skull punch fest. Body to wear them down. MMA you can choke, punch, kick, tackle, and whatever else the body can accomplish to defeat another legally of course. In the street people cant fight like men and punch and kick people when they are knocked out then thats instant brain damage usually. If your lucky you wake up in the hospital and walk out then guess your lucky.
final note..
If you see an unfair fight going on don' let someone just deal with it help them and take out the loser attackers. These people will raise more losers and they are useless to humankind.