People just wanted to be able to talk easily. It was annoying to set up cross platform group messaging at first, but it hasn't been a problem for at least 6 years. The biggest thing pulling people to iMessage now is convenience and not wanting to figure out a different mutual platform.
A large portion of the current generation of kids are actually tied to Discord. I expect Discord to start dominating more and more as time goes on. Hackathons are organized and held on there. Clubs at my university have moved to Discord. There's a specific to my university Math discord and a separate CS one where even teachers interact. A lot of online streamers have set up their own branded discord for their community. I have heard some companies like Amazon even have a new hire Discord. Slack, iMessage, Whatsapp, and Messenger all lack things Discord provides.
I believe Discord is the endgame for group messaging. The way you can organize conversations in to categories and channels is a killer feature that no other service gets quite right. The way Discords use roles to help categorize people into groups and have granular permission control for who can access what content is so useful. It's a matter of when, not if Discord is the best service.
There is one big caveat to Discord right now and that's the lack of End to End encryption. Whatsapp and Signal are the only two messaging services I trust for privacy sensitive communication. I don't really care if government agencies snoop on the fact that I was playing Rocket League for the last two hours though. iMessage is E2E as well but only if you communicate with others that are also tied to the Apple ecosystem which makes it useless for me because a lot of my international friends aren't. I have an ipad that has my contacts synced so I can see who else has iMessage.