Sometimes I think Discord has taken over forums altogether, as well as other forms of chatter. It's faster, more immediate, more satisfying, less work, and mobile chat apps make it a ton easier to respond there than it is to make a large(r) post on an internet forum.

It's been a trend for a while, I think. That, and the other obvious possibilities such as people moving on, losing interest in a community, etc. People who are just discovering ZC and similar communities were probably already too used to the faster chat methodologies so don't get as drawn into forums, or see it as the more difficult choice between the two.