These were all multi-protocol clients supporting more than XMPP because MSN, Yahoo Messenger, and AIM all had their own thing. I believe Facebook was always its own thing but they exposed an XMPP proxy. Only Google Talk really relied on XMPP from the start but it was pretty much a token player in the market until two things happened: 1. They integrated with Gmail making Google Talk readily available to Google’s entire Gmail user base without installing a separate Windows-only client (or finding a compatible messenger, iChat ended up natively supporting XMPP within a couple of years) and 2. Instant Messaging went into rapid decline as users switched to mobile-first messengers (IMing was always a desktop-centric experience and the various pre-existing protocols were not battery efficient and you typically had to have the client open and active to be signed in).

That's not really true. These clients often reversed engineered other chat protocols and thus supported services that didn't support Jabber / XMPP. This was before e2e encryption was the norm so it was easier to do.