Maybe it's because of his animations. GaW's moves aren't fluid animations like all the other characters'. When you L-Cancel with someone else, the animations speed up to finish the attack quicker, but with GaW, the attacks' animations are only a few frames long, so there isn't anything to speed up. (I'm just making this up as I go.) But it makes sense, if you think about it. The nair, uair, and bair attacks are all animated until they're finished, and these are the moves that can't be L-Canceled. However, the two attacks that don't animate all the way through are the same two that can be L-Canceled: fair and dair. Maybe since these two stop animating after the beginning of the attack, the game responds to L-Canceling by simply cutting short the stagnant frame at the end.