A) It taints the perception of the game beforehand. People look at an incredibly raw bit of gameplay footage and go "wow, this looks like trash!"

B) The guy who did the leaking apparently tried to sell the source code, which is just scummy and would likely result in significant retooling from the devs' side, which means time and effort wasted.

C) While I personally don't subscribe to the glamour and glitz, there is something nice about a clean rollout for something anticipated or a surprise release. I remember seeing the first trailer for Peele's Nope and being incredibly excited because I heard nothing about the movie prior to that. If it was leaked or the screenplay was floating around with spoilers, I'd be somewhat less enthused.