Unlike browsers, which can (for now) still be swapped out solely on individual preference, the network effects are real for messaging platforms. This means that currently Signal is more broadly useful than Matrix clients.
Hopefully this can change some day, and Matrix (or something like it) will become ubiquitous. But it was a slog for Signal to gain even its small market share, despite its better usability and lower barriers to entry.
For lack of a better alternative, we "standardized" on Signal as the de-facto secure messaging system that is outside of the control of ad companies, and it will take time to rectify that.
on the Matrix side we are slogging away at https://element.io/blog/element-x-experience-the-future-of-e... to provide good (excellent?) UX as well as interoperability via Matrix.
Isn't the Matrix protocol quite clearly the better alternative to Signal?
There are several options--host your own (matrix server), use a free public server (matrix.org), or pay someone to host a server instance for you (e.g., element.io). The Element clients are good enough by now for all platforms (web, desktop, mobile). What's not to love?