Matrix is not out-of-the-box thing.

For hosting it you really have to go through some trial-and-error before it works as you'd like, and most self-hosting enthusiasts have pretty short span of said enthusiasm.

For users its easier, but there are some idiosyncrasies in terminology, and concepts.

There are docs but they really would benefit from human editing to become fully useful.

Synapse in particular has a problem of existing in two places on GitHub, and the one which is obsolete somehow comes first in searches, and appears in AI responses constantly. Which I guess shoots quite a lot of first tries in their steps.