This is one of those duality things that comes up so often in programming and math: What is optional from the point of view of the user of the format is mandatory from the implementer's point of view. Pointing out that it's optional for the user does not remove the additional complexity the implementer must pay. YAML is a lot more complicated than JSON, and it's not all different ways of spelling the same thing (which is a sort of complexity that at least isolates itself to the parser), it's genuinely richer and more complex, and that price gets paid by somebody. Focusing on the one not paying that price doesn't make it go away.

Further, "YAML is not as popular as JSON" is an observational fact, not something we can argue away. It's worth trying to figure out why and learning what we can from it. I've made my proposal. There are other options ("Javascript is a steamroller"). But I'm not really a big fan of the evergreen "other people are just too stupid to see the obvious truth" explanation, which is generally the one that seems to be given to explain why YAML hasn't beaten JSON. That explanation is almost always wrong. (Only almost, though.)