Because they recognized that there was some value in the centralization, namely that it makes it far easier for users to find content. Putting your own blog out

Because they recognized that there was some value in the centralization, namely that it makes it far easier for users to find content. Putting your own blog out there into the ether is far, far, far more difficult to drive users to it. And then, if you want to have comments, now you have to invest time and effort into moderation and anti-spam, whereas here Medium takes care of most of that.