It wasnt until MK3 that Shao Kahn invaded earth and his extermination squads injured Kabal leaving him alomst dead....but he wans dead,he was alive but horribly disfigured so he must use the mask,that isn't exactly a mask,its an artificial respirator to keep him alive.In Mortal KOmbat Shaolin Monks you see Kabal without mask because MKSM storyline revolves around MK and MK2,before Shao Kahns invasion to earth,and MK:Deception is after MK3 so you see Kabal with the respirator.

mk:sm was in between 1and 2, and it was in three that he was scarred by the extermaniation squads.

A simple timeline . . .

MKSM= MK2 based

MK2= Kabal was a normal human with no mask (even though he wasn't in MK2).

MK3= Kabal was scarred by Kahn's extermination squads, therefore needing a breathing mask to keep him alive.

MK3-MKA= Took place after MK2, even though Shaolin Monks came out after MKD, so that is why you see Kabal with the breathing mask.

really?ok thanks.i was wondering that

Yeah, Reptile88 and RaisnCain are right, about Kabal's mask, with the extermination squads. Well, since the question has been answered, I think this can be closed.
-RyanRaiden.