I'm seriously considering moving to Reddit, where they actually have people who care.
Most Helpful Girl
I obviously can't speak for the admins but I imagine that it's because very, very few people on this site will have medical or therapeutic experience and are not qualified to deal with a suicidal person properly. Even though they may care, they may actually make the situation worse, perhaps through coddling the person rather than supporting, or through accidentally saying the wrong thing. There are also people on this site who will post purposefully upsetting things to anyone who confesses to be suicidal which possibly could push them into hurting themselves. I imagine that we're asked to remove these things for the safety of the person in question.
I've also seen that the site sends people who have suicidal posts removed a list of helplines or something similar where they can seek proper care that the people on this site are not trained to give.1