Why zulip instead of the good ol' IRC?

It has modern features. It stores message history. It has a fairly unique feature of letting you create ad-hoc "topics" (that go under a "Channel") that make it easier to manage the flood of conversation.

Channels + topics >>> just channels

1: IRC loses all messages to you while you are not connected

Not for years. If that is still the case for you, ask your server hosts to update to a version that supports ircv3