My feeling is that Discord is taking over Slack for a simple reason: it's easier for "community contributions".

In Slack, if you want to keep the history (which most people seem to really, really need for some reason I don't get), the owner of the community needs to pay (a lot).

On Discord, the community members seem to be able to make some kind of contributions as a "donation". Not sure exactly how it works, but I have seen a few open source communities move to Discord for this reason.