Isn't comparing the scale of the two like comparing apples to oranges? I think most wordpress sites are self hosted. There's wordpress.com hosting, but I'd guess that most of their sites are self hosted for the explicit purpose of customization.

While WordPress still dominates large areas of the web, they have a decentralized model vs medium. It's like comparing scaling web forums to reddit ("why do they have so many outages? All they do is store text!"), but reddit obviously has a way higher load than any single forum. I'd wager the same is true for medium.

To continue on with the metaphor, medium, like reddit, is solving a different problem than the wordpress/squarespace model: you don't have to roll your own. You get a built in audience due to centralization and the cache of the site (For some reason I associate a medium link as being more legit than seeing a WordPress hosting site. Strange, but better branding I guess.)