Basically, because there's no alternative for a truly open, striving for security messenger software. It's imperfect, but, honestly, Matrix/Riot is as good as it gets for now. Both server and client software are open-source, and it is designed to be federated network, meaning if you don't trust their e2e encryption or whatever, you can just roll up your own server and use it to communicate with somebody. And it's pretty decent in terms of features already implemented, it's easy to write helper-bots, it has clients for all major OS. And, sadly, almost everyone still uses Telegram/Whatsapp instead, which is, I believe the main reason you (and everyone else asking) should start using it even if you don't feel like you have to, because none of your friends are currently on it.

I'm still very partial (obviously) to the Introduction to Matrix guide we wrote as part of Ruma's documentation: https://www.ruma.io/docs/matrix/

It's xmpp reborn (or IRC if you're that way), with a modern feature set, so the thing you use if you want Slack but without SlackCo.

- no vendor lock-in

- decentralized

- encrypted/private/secure

- open-source

It's like irc or email for private decentralized encrypted chat.