Because they didn't reinvent xmpp, they reinvented Google Wave.
A lot of Matrix.org is inspired by what Wave could have been - although we are running late with threading (but some work is happening currently there). From our perspective, one of the biggest gaps was the lack of bridging in Wave to other existing comms interfaces (eg email) - as well as the quirky UX.
Why don't you ask the Matrix team why they decided to re-invent XMPP and add a stupid HTTP API?
@onlinepersona
@netchami
Because they didn't reinvent xmpp, they reinvented Google Wave.
Arathorn on Feb 18, 2018 on HackerNews
I take it you weren't aware that Google Wave was based on XMPP.
https://www.networkworld.com/article/757722/opensource-subnet-nine-new-apps-that-show-off-the-power-of-google-wave.html
@onlinepersona
Thank you, I did not know that!
Very interesting.