either because their pretty, or have a funny/likable personality. even by saying that, it still confuses me why company's don't want a group balanced out in talent, they want one or two members who struggle with a lot, just to have them weigh down the group? the thought process is wild. something to remember is that the training system is very rigged towards richer trainees, and even if they have no talent, they can still be put into the group.
They don't want anyone weighing down the group, they want members who are passable but will not outshine the chosen ones. I could give numerous examples of Kpop companies being guilty of favoritism. They usually already know who they want to make into a big star by the time a group debuts and who is just filler. You don't usually see someone who is actually completely useless debut, but when it does happen, they usually have connections of some sort.