I think it's about service that are stream-centered and sectioned.
HN, Reddit, Upbeatapp, Quora (?), are stream-centered. Information come and go, which make it very easy accessible.
Reddit, Upbeatapp, Quora are also sectioned ; There are sub-reddit, there are subgenres in Upbeatapp and in Quora, you can follow topics/interests/questions.
HN is not sectioned but is already very specific.
That point make an access to your pre-targeted audience.
The two factor combines make an easy accessible access to a large interested audience.
( Note that is not the case in twitter, youtube, soundcloud, tumblr...)
I don't know Medium enough to be sure, but I think they are trying to do the same (sections + stream).
I'm trying to find online services that use this pattern for other type of market (Movie, Food recipes, ect.)
Please elaborate on that thought!