I think the "thread-only" feature is the biggest selling point, it's niche-y by default.
Zulip is the meeting point of a hackernews/reddit-like board, real time chats and mailing lists. UX and UI needs work, but the base concept is its main feature.
I participate in two different zulip's, the microsoft rush one and the functional programming one and I always felt Zulip was the best choice for technical discussions.
Threads discourage spam and off topic, searching discussions and topics you care about is simpler. If you come from 3 decades of IRC it gets time to get used to, but it's great in everything IRC is unfit.