In reply to Sally G..

A few reasons:

1) WordPress being an open source project would benefit from an open source chat system. We would not rely on a 3rd party for our real-time communication.
2) Reduces on-boarding friction for new users. A common complaint is wasting significant amount of time on contributor days to onboard first time contributors. With Matrix, you see the chat right on page, without even requiring an account & can login for participation with just few clicks.
3) Bring together all local WordPress slack communities, which mostly don't have full retention of messages being on free plan.