Re: Quick - call a sociologist
Ease of use. That's the simple one - As someone who has been running dovecot and prosody for a while - I more or less gave up on xmpp - it's very clunky it doesn't do stuff as easily as it can - certain things like message history is lackluster etc...
Since I'm a gamer I've started using discord and I can put it in simple terms:
It's easy to setup, it has oth group chats and servers - basically it's an improved IRC.
Need to share a file with a person or group - attach it to the message. It's there.
The feel of it is not that far from IRC as well. Of course if you prefer you can go with a more "chat" like feel as well. It gives you that.
There is no "setup an account on some xmpp server" - it's simple - create account, use existing email, start using it.
The user experience is what counts now-a-days. This is the main reason people go to something. They try it out and start using it. Then invite friends.
An example from a friend that only recently started using discord - "This is what I like about WhatsApp - I can tell when my friend saw the message."
Honestly even users themselves often don't know what that killer feature is that they want.