YAML Versus JSON

YAML is technically a superset of JSON. While YAML is more human friendly than JSON, JSON is easier and faster to parse. Therefore, it is recommended to use YAML for configure files, JSON for APIs. And that is why MongoDB choose YAML for configure file while it already has to parse JSON in the first place.

JSON is much less human editable than YAML. It is also a little less human readable than YAML, although it much better than XML. JSON does not support comments, which can be really important in configure files. Complex datatypes is also a strength of YAML.