The competitors to Zulip are Slack and Discord, maybe Matrix/IRC. The question is how do you group subjects and sub-subjects aka threading and Zulip forces the user to pick both explicitly. This results in an easier to follow conversation. Eg in the 3d printing channel there's a modeling thread and a filament thread, so if you want to see stuff about filaments, you don't see the conversation about modeling unless you specifically go over to that thread. Meanwhile, Slack's threads are part of the channel, so you can go into a channel, post a message, and half the conversation is in the channel itself, and half the conversation is in a thread. So you're forced to read all of slack to keep up with the conversation. At the end of the day, it's about signal to noise, and Zulip's UX raises signal.