Besides, there's the convenience and ease of use aspect. Most of these federated services are not easy to use for the average newcomer or tech-illiterate user demographic, for example. And sometimes they're inconvenient.
Compare:
"wordpress.com" (hosted): "Create your new website for free. WordPress.com is the best place for your personal blog or business site. Create Website (button)"
"wordpress.org" (download): "The latest stable release of WordPress (Version 4.5.2) is available in two formats from the links to your right. If you have no idea what to do with this download, we recommend signing up with one of our web hosting partners that offers a one-click install of WordPress or getting a free account on WordPress.com."
Open source just doesn't get user-friendly onboarding.