https://zulip.com/values explains how we are building Zulip to be sustainable for the long term (without VC funding).
Zulip is open source and you can self-host it.
It should be obvious that Slack has the usage it does specifically because the groups you mention don't want to take on the effort and costs of hosting and ongoing maintenance.
They have a cloud offering with import and export tools, so one can still use that and migrate to self-hosted when or if they want.
Your question was why is Zulip better; I'd argue that's a pretty strong reason.
Zulip's free plan allows 10,000 messages of history and 5GB storage, similar to (and arguably worse than) Slack.
That's what slack used to offer, and it was actually usable for small communities.
90 days is useless for basically everyone.
We (Zulip) also have a free Community plan that we sponsor for thousands of communities, that does not limit message history.