Hmm I suspect that the writers wanted to keep the adult storyline going because they're the biggest ~names~ of the cast and add a little more kudos to the show? Maybe they wanted to avoid having a cast with solely teenagers for that reason, although really if the entire premise of your show involves 100 teenagers battling for survival then you may as well just wholeheartedly commit to it, I doubt the snobbier viewers will look any differently on the show as a whole whether they include a few older actors or not