As someone who has self hosted my own XMPP server before and still cries over the move taken by Google and Facebook with their messengers, can someone from the Matrix people explain what is so fundamentally different from the protocol, that it couldn't be done in XMPP?

Please don't say "it is XML based". I know the situation with the clients is XMPP-land is appalling, but it seems to me that if Matrix does get any kind of traction, it will face the exact same kind of problems.