Because if you train for self defence I suggest that you disregard alot of UFC video footage, UFC is not self defence, although it can get the closest to it bar R.B.S.D. Taken the way that you train, a top UFC fighter could get his *** kicked in a street brawl whereas a traditional MA who trains purely for self defence could come out relatively unharmed, but then again circumstances would tell.
UFC isnt so "Ultimate". However, given someone should have, could have, would have, is all speculation on what had happened. given that it is one bout in a environement that was suited for one competitor and not the other.
Just dont get put off by some video clips, how do you know thats really a WC guy, might just be a random guy dressed in a Wing Chun suit or tee shirt.
If it is, or isnt, the guy had a lot to learn from defeat.