From practice - performance of clients and of servers too.
From emotion - it uses Web technologies.
From some logic maybe - if they are doing something new, then why not distributed architecture like Tox (at least identities not tied to servers), and if they choose something architecturally similar to XMPP, why not use XMPP.
However, emotion again, I really like Matrix APIs, these are definitely designed to be used by anyone at all.