Medium is only an edge server of your POSSE CDN, your own blog is the origin
People tend to be lazy (I am often) and chose the easiest tool to do the job, even if it's not the best one. Medium is the new Blogger, but with a much nicer content editing UI, and a cleaner presentation of content to readers. People can create and account and start publish in a few minutes, it's really easy. But how can all this be free?