And none of them actually work really well. This blog post is over ten years old, but the situation didn’t improve much https://matej.ceplovi.cz/blog/current-state-of-the-distribut...
It may be that the presence of Fossil as a mature option is enough to dissuade people from choosing/working on alternatives. I’m pleased to see git-bug slowly making progress though, maybe it will get there in time.
I looked into them. Most of them use some CRDT-ish machinery to merge concurrent ticket changes. (Fixing merge conflicts in tickets would be too disappointing.) As a CRDT person, I noticed that implementing CRDT merges in git itself mostly makes that machinery unnecessary. Then you can keep your tickets in plain Markdown, having CRDT merges for both code and metadata. I have it working here btw