One reason for the move not mentioned in OP,
https://reactjs.org/blog/2015/10/19/reactiflux-is-moving-to-...
"Discord is the easiest platform to join. New users can immediately join our conversations without having to create an account."
That is, frankly, the killer feature.
Notwithstanding, I find it odd that developers here don't use E2E chats. It's always something clear text with permanent history, and in the case of Slack, key logging too! There's a JS based xmpp framework out there.
It puzzles me why no one uses such things.