Adium was a total lifesaver on Mac since there was never an official MSN messenger client there.
Back then a lot of services also used XMPP but that has been readily abandoned - Slack used to do XMPP, doesn't any more. Google used to do XMPP, doesn't any more...
I think their switch away from XMPP predates E2EE and some of them still aren't using E2EE, so that's not the real reason. XMPP has good E2EE now with OMEMO though.
Back then a lot of services also used XMPP but that has been readily abandoned - Slack used to do XMPP, doesn't any more. Google used to do XMPP, doesn't any more...
I suppose one reason for it is E2E encryption.