One answer: eyeballs. Medium has managed to bring audiences and audiences bring good writing and money (both to Medium and the writers).
The thing with big sustainable audiences is that there has to be some level of centralization, either of hosted content (medium), or linked content (google).
You don't need a hosted service like medium, but you do need some way to bring the right content to the right people. To be honest, even with great insightful writing, I doubt a single blog can tackle this task. In fact, I think the biggest blogs are big not for the content, but for the marketing/leads generation/traffic building skills of the owners.