Have you tried... thinking about it a bit more? I could come up with roughly 20 different reasons without trying very hard. For someone involved in XMPP, that is one hell of a thing to say.
Seriously, calling out XKCD927 on a massive project like Matrix is extremely disrespectful to its contributors.
Tell me, what could Matrix have done to save XMPP? Publish a hundred different XEPs nobody would ever bother to implement? Make broken clients/servers that don't follow the spec just to try fix a protocol that lost? Magically fix everything and tell people "No really guys, XMPP is better now I swear!"?
They went and tried a new approach and they're doing really well with it... have some respect for that.