The reasons I like yaml is that I can see the tree structure directly, and to my lisp brain it is extremely easy to read. Furthermore, in our age of LLMs, I find LLMs to be able to generate "correct" YAML more easily than JSON, since the tree depth is encoded in every line, and doesn't require matching larger structures. It also uses an actually significant amount less tokens.
I find it extremely easy to have LLMs generate decent DSLs by asking them to use a YAML output format, and found it very robust to generate code out of these (or generate an interpreter for the newly created DSL).
I didn't know about !tags until sunday, which is quite shameful, but I find that the emrichen solution is actually quite elegant, and really kind of feels like a lisp macro expander.
Overall, YAML is just good enough for me to get shit done, I can read and skim it quickly, LLMs do well with it, and it's easy to work with. It has aliases, multiline strings and some other Quality of life features built in.