> I am genuinely interested in hearing why we collectively ditched XMPP

We didn't. It was never very popular, and is today more popular that it has ever been.

Depends if you mean just the technology or using it in the small federated spirit. Google Talk and Facebook Messenger were XMPP all the way through and worked with vanilla XMPP clients. Slack wasn't XMPP but supported it via a gateway until it was dropped.

Not sure how popular the small federation was back then, but I know Mac OS X Server touted an XMPP server and that was a first-class feature of iChat.

It wasn’t popular? I remember using pidgin to talk to friends on google chat, facebook and my work contacts. It was glorious.

I haven’t had a reason to use an xmpp client in over a decade.