Popularity is irrelevant imo, it's a general purpose FOSS tool. Ignore Element and matrix.org, the open protocol is what matters. These features are useful even if matrix never gains any network effect: 1. talking among a small-ish specific group that needs sovereign communication, e.g. when forming a company or a tight knit in person friend group 2. only data limit is the size of my harddrive 3. bridge to every other popular protocol 4. personal bots and automation with no restrictions 5. Unlimited and customizable clients with no restrictions 6. Combinations of 1-5.

Can't speak for non-self hosters or people who aren't serious about chat, but for chat enthusiasts who can setup a server with bridges and bots, matrix is incredibly useful.