Why can't I delete both account and content when cancelling a user account, in Drupal 8?
Deleting content requires an access check, and as such the Delete the account and its content. option can't be selected as the default. I presume this is because a user can have access to cancel their own account, but not be able to select the method, and there's no mechanism implemented to provide a fallback for the default.

You'll still see it when cancelling an individual account, provided you have the administer users permission:



If you'd rather get control of that process yourself you can implement hook_user_cancel_methods_alter to remove or change the access check.