I answered this question a while ago in a post in my dashboard. I would like to share it again as my own post.
As far as I have observed in the KPOP world, entertainment companies seek out to have a role for each member of an idol group - there are the vocalists, rappers, “image-carriers” (maknaes and mascots) and the dancers. I could probably compare Kiseop’s situation with Hyoyeon’s of SNSD. (Of course, you are free to disagree with me. This is just MY point of view.) If you have also noticed, Hyoyeon doesn’t get much lines in a song, yet she still remains as an SNSD member. It’s because she is seen as more of a dancer and that what makes her a unique member of the group.
As for U-KISS’ case, the vocalists are Soohyun, Kevin and (now) Hoon. Eli, now with AJ, are now the rappers. Although we are suprised that Dongho can rap really good, he actually plays more of the “image-carrier” of the group. So where does this leave Kiseop? Yes, he has a beautiful voice, but it isn’t as strong as Soohyun’s, Kevin’s, Kibum’s or Hoon’s. Yes, he can act but not as expressive as Eli’s, Kibum’s or Dongho’s. (I don’t know if he can rap.) What can make him stand out is his dancing, and he probably knows that.
It will be a mystery and a heartache for all of us on why NHM has made such a decision.
PS: I want everyone to understand that this is only my point of view. You are free to disagree.