Are you guys serious? lol Liu Kang a vital character? He hasn't been essential to the series for a while now. People are sick of seeing him win every tournament and when Liu Kang isn't winning... He's irrelevant in almost every way. He is not a character with a lot going on for him. 20 something years later, I think the fan base is ready to let go. I rarely ever see people raving about him. He is one character that has overstayed his welcome. Same for most of the originals. And with the story jumping so far ahead into the future, it just seems idiotic to keep these characters on board. NRS is trying to build the next generation of MK heroes. There's no place for Liu, Johnny or Sonya in that future.
Enter characters like Cassie Cage, who replaces someone like Sonya without straying too far from the MK norm. Seriously though, Scorpion and Sub Zero are the only must haves at this point. They could even get away with excluding Raiden. Which I predict will happen very soon.