With the original full face mask (the first one he wore), I think Kane himself wanted to ditch it as he was having trouble breathing and was too hot. He then went through a few other masks/half masks after that. Like Bodom said, getting rid of the mask for good was probably just time for the character to evolve, and it was a pretty big thing at the time. There's always scope to bring the mask back (through an inferno match (if they ever bring that back), or if he somehow gets scarred in a match). Even though a lot of fans would love to see the mask back and the scope to do so is there, I doubt it'll happen. Kane is 43 years old and probably thinking of retirement within the next few years.