Drupal 8 uses YAML* as its configuration language because JSON doesn't support comments. That simple. Thank you for YAML, it does deliver for us: it's human readable and it's easy to parse (see below).
* I mean, it uses an ill defined subset of YAML. The definition is "whatever the Symfony YAML parser supports".
* I mean, it uses an ill defined subset of YAML. The definition is "whatever the Symfony YAML parser supports".