Probably because the mods got told by the Reddit higher ups that social sites like theirs and Discord are going to start being watched by Homeland Security after the last 7 mass shootings the news propped up were all carried out by people who had strong ties to websites like Reddit and Discord. So a post talking about shooting people and getting voted up in a front page sub will only serve to give people cause to point out that these sites are celebrating the violence.

So they leave it up long enough for people to buy Reddit Gold so the company can get a cut of profit from popular posts then curtail it without ever actually creating a policy that would negatively affect these major subs full of idiots.