I'm 90% sure it's because of the voice actor strike. So far it seems the voices for Scorpion, Kitana, Mileena and potentially Sonya Blade are gone.
Phil LaMarr (Kotal Kahn) and Ashley Burch (Cassie Cage) were apart of the strike. I'm not 100% sure, but I think that Steve Blum (Sub-Zero, Reptile, Bo' Rai Cho) was apart of the strike as well. If they get punished, then we could be losing at least 5 more character voices.
I will note that LaMarr and Blum returned for Injustice 2, so while it may seem like they are not out, it really depends on how their contracts work. I know work on Injustice 2 started before Mortal Kombat X was even out yet which was in 2015. If they started doing voice work for the game mid-2016 then that was actually before the strike happened, which was in October. They definitely got contracts for Injustice 2 before they started the strike, now that the strike is done and over, they may not be back for future projects.
The only thing I can think of to go against this is that Blum did Sub-Zero's voice in Injustice 2 as well, and he was released months after the game's launch. Unless he recorded his lines for Sub-Zero around the same time as Green Lantern (for the base game) -- which I feel is somewhat unlikely -- then he must have returned for additional voices. But again, since he had a contract before he went on strike maybe apart of it was that he was obligated to do more voices in post launch or something, I really don't know. So even then, it's possible he was only able to do Sub-Zero's voice again because of the fact he was able to get the contract before the strike.
We will have to wait and see, but it's not looking good for fans of the original voices. It really sucks for the voice actors involved, but with that being said it doesn't mean that the voices for these characters will be bad or generally not done justice. If it's true that they are getting Ronda Rousey for Sonya Blade, then honestly it's not really a strong way to start things, but here's to hoping that her voice is good and the other actors/actresses they get are as well.
EDIT: Looked it up and it does seem that Steve Blum was apart of the strike. He even had a site for a limited time promoting the strike and going over information detailing what it was all about.