Well, I'm actually not surprised.
I think the first sign was when he decided to rewrite part of it (the first scenes) and that the whole thing was way too overplanned, which can either lead to a successful product or, as in this case, to hitting wall after wall until you get completely stuck and drained by your own project.
Myst was a good game because he had fun creating it chapter by chapter, without huge plans behind it, or at least that’s how I remember it.
There’s nothing to be ashamed of. I think it’s really mature to consider the personal side and how this would have affected his finances. Some devs decide to keep going and milk it for years, so again, it was somewhat a mature choice, even though a risky one, as abandoning something is always a double-edged sword.
Wishing all the best for the new game and maybe it could also give some inputs for continuing this one.