The main reason is down to tactics, each fighter is usually fluent in at least 4 martial arts and grappling disciplines. Normal street fights are pretty much like boxing and most people dont think of alternative moves etc, its usually tit for tat punches, bottle of bud accross the chops etc. Like boxing, all you have to worry about is getting punched. With MMA, you have to worry about punches, kicks, sweep, grabs, and the worst is beign taken to the mat.

The main tactic is taking control of the fight, not knock them out, thats why a lot of mma competitors will try and get close to the opponent and on most occations take them down to the canvas, once they have control thats when the quick finish happens.

I love watching mma as there is so much involved.

Check this link out, every knock out shows one opponent getting control, once they have control its over in seconds

http://www.youtube.com/watch?v=uCwgQmulx2E