in slack and discord i don't need another app i just hit the huddle button (in slack) and join a voice chat room in discord.

This is the problem with pitching zulip to this audience. The original thing that got gamers to switch to discord, it was their original and probably still is their primary target market, was a single login to a huge universe of voice chat rooms. Before discord gamers were setting up/renting teamspeak and ventrillo services (that were voice chat only). Hell for the first couple of years of using discord with my gaming group the only thing anyone ever posted in text was what time they were going to be on and what game they wanted to play.

I would say the main thing that got people to switch to Discord was the fact that voice chat was free, paid for by VC money.

sure but also that you didn't have to pass around your teamspeak/ventrillo information to every rando you wanted to play with, less friction in discord with invite links etc

I have never created an account nor logged on to use jitsi.