My “why not Matrix” is that it’s hard to build a good client for it and hence there are incredibly few (on some days I’d argue none).

To the project’s credit: A lot of work has gone into supporting better UX (QR code sign in, sliding sync) and the Rust library is also apparently pretty good.

Yet, somehow, there still hasn’t been an explosion of clients. IMO it’s because the protocol carries too much cruft and the standardization process (including vendoring) makes it hard to use new features when not using the official libraries, which are a) under-documented and b) only available from Rust, Swift and Android.