Cos he's an incredibly ugly bastard.
And cos it's always tradition for the good lucha guys (tecnicos) to have their masks and never take them off. However, this is the same case for the bad lucha guys (rudos), who will usually try and "expose" the wrestler for who they really are, If i'm not mistaken, removing the mask is basically the Mexican equivilent of a low blow. It's illegal to remove the masks.
Many lucha libre wrestlers will go to great lengths to keep their identities a secret. Case and point, one of the most popular lucha libre tecnicos', Mistico, currently working for CMLL. No one actually knows his real name, or who he is, because of the incredible length his gone through to have an almost "secret identity." You also see that in lucha libre, when ever they lose for example a hair vs mask or mask vs mask match, the loser will never wrestle with a mask again, because they respect being the loser of the "bet" and that it is agreed that once you loose your mask, you don't just turn up the next week and go "Surprise, it's just a storyline."