I think it's the pressure and the solo developer model

You put something out. It seems to be running. There are bugs. You fix them. There are more bugs (pareto is a PITA). You fix them too. Someone accuses you of sacrificing kittens in the basement. There are ideas for features and more bugs. Some worded in an entitled or just tone-deaf way. Very often you're the only one working on the project and saying "I accept PRs" somehow became received as a FU. It becomes hard to put the project away for just a week
You are perpetually in "I-fix-it" mode
Not to mention that in order to even start a project like that one needs to have at least a bit of "fuck it, I'll do it myself" mentality

It's a pity that !unfinishedprojects@forum.unfinishedprojects.net wound down. I hoped it might have been a space for projects to find communities. As we see around, it doesn't require a 10 person team with a kanban board to write something that works. But on the other side of the spectrum I think a project needs some sort of community around it. So even if there's only one dev, someone else could write the tutorials, less technical parts of docs, help new deployments etc