Your analogy is flawed. There were and are plenty of other "create a blog with a few clicks that you don't have to host" services.
It's a blogging platform. A good one. Someone wanted to make it, and he built it, and now it's successful.
Maybe it didn't revolutionize the way we make toasts, but it's a solid incremental improvement over the alternatives. So was facebook, so was iphone, so were most of the other things people have made.
Namely "Medium makes it really easy to post something that looks good across devices etc., and has good sharing integration"