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.