No seriously, what's the value proposition?
Youtube at the time had an awesome "free" service when most others were littered with ads and had terrible video players.
Instagram had an awesome mobile service that was hip and clicked with it's users.
Medium is at best a crappy bloggr that was once pretty awesome but now that it has a lot more users (and worse a lot more writers), the quality of content has dropped significantly.
I've read articles about from people who are basically using Medium to do content marketing and worse yet, do dumb challenges like 1+ article / day challenge - case and point: https://medium.com/@larrykim
Medium is quickly turning into hipster 9gag or buzzfeed.
I'm definitely not saying we don't need a Youtube for written content, but imo, Medium is not yet it.