I think that’s very much contextual. Zulip was tried at a time when there was much less demand or perceived need for real-time communication among Python core devs (the #python-dev channel on IRC was similarly relatively idle).
For reference, here is the 2021 poll/vote for a core dev chat choice. For some reason the stated result (“82% of us have voted for a Discord Server”) doesn’t match the poll results in the thread (where Discord arrives third after IRC and Slack, while Zulip wasn’t explicitly considered), but I might be misreading that part.