Because privacy and convenience are two extreme opposites and you can only go so far in the privacy direction before you start losing everything. Discord just works a million times better as a public forum/community than Matrix and is much more easily accessible to everyone.
There is a limit. I am privacy conscious but I still use all Google Services for example, because they actually provide me with a better web, work, mobile and entertainment experiences. Similarly, I prefer Discord for big communities with channels, server bots and topics, over Matrix.
Edit: all those people saying we can’t be privacy conscious and use Google Services at the same time: yes you can. Their services literally make my life better so I will keep using them, but I keep what I share with them to the absolute minimum. I go into their settings and disable everything I can about tracking and ads personalization (even if they still track me, I do my best not to be). You can surely still be privacy conscious using non-private products. Being extremist is not how you convince average joes to think about privacy, nor by telling them to give up all they use for unknown (for them) alternatives.
And yet people will just shrug it off and keep using windows. And Microsoft loves that.