I imagine it's because those trolls still have no consequences to their actions. If people would make more public the comments of those who are the most toxic (death/rape/other violence threats, etc), by, say, sending those comments to the person's mother/grandmother, their boss, etc, and they actually had consequences for those actions, then it might taper off.*

* Of course, this kind of action can have some far reaching consequences, and is quite ripe for abuse. I don't know if there is a way to fix that. And myself, not being the target of such abuse online, I'm not really that invested in trying it.