How do I add a menu item to the main navigation from a custom module in Drupal 8?
You are missing the "menu_name" key as said on : https://www.drupal.org/node/2122241

this example should work : 

demo.demo:
  title: Demo Link
  description: 'This is a demo link'
  parent: main
  route_name: demo.demo
  menu_name: main