They just want to make extra money on top of the game. The fact that they are already planning on releasing DLC for these fighters tells me that they more than likely already have the character models, they just want the user to buy the game, and then buy the extra fighters.
I will buy the game and probably buy the DLC eventually as well, but it is pretty BS to not just put them into the game.
"Don't be shocked that people die. Be surprised you're still alive."