They decided to up their price per user by a significant amount not too long ago... our self-hosted instance suddenly became more expensive than Slack but with an obviously not nearly as polished product as Slack. So we moved over to Zulip because at least they do their own thing, we haven't had any regrets over the switch to Zulip.
It was frustrating because Zulip felt like it was so close to being a great - and unique - product. Might be time to take another look and see if it’s there yet. It’s a really great community, friendly and very responsive on GH issues.