Johnny became extremely popular after MK 9 and his daughter was well received in X. Sonya's been a fan favorite since her debut so NRS pushing them makes sense.
spoilers:end of spoilers
Also, Liu Kang ends up being the one who defeats Kronika in the story's climax. Not to mention Kitana killing Shao Kahn.
I liked Jacqui in X but she didn't get much character development in that game compared to this one.