cuz they are greedy bastards. The Versus mode was completed and already on the disc when the game released. Capcom just decided that people were suckers to shell out an extra $5 for something that is standard and included in other games.
If you are talking about the update back in Feb then the only thing they change was that they made to easier for the Hunter to pounce on a survivor who is meleeing and increased the minimum damage it does when it pounces on a survivor. If are referring to the update in April then no real changes just adding Survivor mode and maps to game.