Drupal 8 Configuration API - cant access variables
To elaborate on @dawehner's answers, default configuration is not cached.

It is only imported when you install a module. After that, if you make changes to default configuration, the only way to add it to existing installations is to write an update function that adds it manually.

During development, there are modules like Configuration development, that can automate that process, but they will obviously not be present on any productive installations.