Maybe also to add to that: If you start having things like arrays and dictionaries available, people will start using them in their configuration and then configuration starts becoming an issue. I don't think I've seen a product where the configuration is in YAML or JSON, where the configuration files haven't grown into a confusing mess.
- xml is too verbose
- yaml is too complex + suffers some notable ambiguity issues
- json isn't very human friendly (no comments)
- a lot of other linux software uses ini-style configs