I'm curious about this too. I went to Lee Valley one day to buy a C12 but decided to get a C15 after noticing the two drills were exactly the same size (without batteries). Maybe too many other people did the same?
I noticed someone talking about this drill being discontinued on the forum a while back, I can't remember what post either. LeeValley still hasn't pulled the C12 off their website yet, not sure why either because they don't have any in stock and the stock never seems to replenish. That's the extent of my knowledge on the subject.