Maybe they realized that in a small companion pool Lae'zel and Karlach are too similar? Exotic race and background, strong warrior women, similar attitudes, martial classes...
They are hardly that similar - on top of that Karlach's story will tie with Hell, which seems to have some part to play with Rafael, and our initial visit. Everything points to her being a future companion - even in curent build she said that as a reward for helping her she will be a friend that will crush skulls of your enemies, or something like that.