SEO would be one theory. Pages under the same 'www' subdomain will provide a more positive SEO boost to medium.com than pages under various subdomains.
Re:The making of Medium.com
by thoughtcriminal on Thursday May 30, 2013 @09:44PM
(#5796394)
It's not annoying for Medium. It's free labor.
If nothing else it sure makes posts from Medium very annoying on HN because I can't actually tell which blogger's post I'm going to read until I click the link (primarily read on iPad so no hovering).