That is what I thought as well, I also have no idea why Medium is so popular still aside from the initial hype of being "a good experience" for writing.
I haven't used it, but I think the appeal was that by posting to a place with a pre-existing audience, you could catch traffic from an initial audience that you otherwise would not have.
I think Medium was the alternative to posting to Facebook, LinkedIn and Wordpress.com. In that sense, it doesn't need a killer, as it's the one dying under its attempts to kill.
