I don't like it either, but I'm not entirely sure why. Maybe the biggest flaw to me is it uses Python style indentation for structuring as part of data logic. It doesn't feel like a configuration language to me and it does not feel simple too. It's also unlike most programming language structures (maybe besides Python), so it looks weird to read and write. Other than that, I don't know exactly why I don't like this format much. Admittedly, I did not do much in YAML, so because lack of experience take my opinion with a thick grain of salt.

We have JSON and TOML. I quiet like TOML. We have "better" alternatives, that are probably easier to parse. And therefore there is not much need for YAML. Maybe if YAML was the default config format for Python it would get off the ground and be accepted more often.