Having written some XMPP code in xmpprs, I can assure you, XMPP is not the best.

With the bridges to other networks, you can pretty much talk to everyone from inside Matrix regardless of what they use.

Then again, the DMA should kick in next year, at that point big players like iMessage, Whatsapp and whoever else counts as big (so probably not Google Chat) will have to allow interoperability between everyone. So they are going to be forced to allow you to somehow massage someone from Whatsapp through Matrix and group chat with people in Instagram and Signal all at once. Matrix has been suggested as the backbone for this interpretability.

Isn’t Matrix the preferred instant messaging protocol for fediverse users?

For the same reason that I don’t use Reddit as a link aggregator.