I think a better title & question would be, "Why is Mastodon struggling to thrive?"

It's surviving no problem, but it's not thriving for a multitude of reasons. Some are pretty well covered across comments here & in the linked discussion, and are more or less reiterations of prior discussions on the matter.

Ultimately I think as much as many of those reasons are correct, the biggest reason is the same as ever: network effects. All the jank and technical details could be endured and adjusted to if there was sufficient value to be had in doing so, i.e. following accounts of interest/entertainment, connecting with friends, etc. That's proven to varying degrees by those that have stuck with Mastodon. In turn, however, it's also clear by how many bounce off that for many there's still insufficient value to be found across Mastodon instances to justify dealing with all the rough edges.

If Mastodon had enough broadly appealing/interesting people/accounts across its instances, people might deal with the various technical and cultural rough spots the same way they deal with similar on other social networks they may complain about yet won't leave. There still aren't enough of those sorts on there for many though, so Mastodon simply survives but doesn't thrive.