Crusher and Spitter are "genetically engineered" by the Alien Queen or by other aliens (Via some sort of "Royal jelly type substance) to handle certain strategic difficulties in battle. Need to bust open a big door or part of a building? Get a crusher. Need to snipe out some folks in an open area? Get a spitter. Let's say the Aliens queen didnt feel the need to do stuff like that so.. they didnt show up in the movie
because they werent there at all. In Aliens, they appeared to have a neat strategy all figured out. Wait in the shadows and attack them. Use the ceiling to navigate the colony and surprise attack the marines etc... In a way the aliens in that film seemed to be going through the motions.. just repeating strategies that worked before with the colonists... in the game they have to handle larger teams of marines... so new strategies are in order.