SimpleX isn't a fun or customisable messenger, so most people will not use it until it has the other features that Telegram or Signal have. Its funding sources/business structure are suspect, it could easily end up sold off and/or enshitified.

Also, SimpleX doesn't have easy or in some cases possible (as it isn't possible to access the file structure completely on some OSs) profile/message syncing on all platforms.

Finally, SimpleX isn't fully compatible with all devices and sticks them in an awful looking mode with no prioritised plans to fix this, Telegram and a few others don't have this problem.

Briar isn't on all platforms yet, so that's why there.