Yes I was curious too, Joe Rogan explained it recently I believe, saying that the UFC decided to make then exhibitions so they dont' have to report the results (wins and losses).

That way it isn't public info on who won and who lost before the season starts.

It doesn't ruin the show that way and it doesn't go on the guys record so I'm sure it further encourages prospects to go on the show, tho I doubt they ever needed any extra incentive due to the massive success and exposure of the show.