As others have said, generally, you're competing with other Medium writers and readers for attention.
The ideal is to get additional readers coming in from outside Medium, who then sign up and read your stuff, but that's not easy.
Overall though, you're right that it is a different entity and SEO is not the deciding factor for success like it is here at HP.
Sensational, dramatic personal stories, controversial opinions, as well as clickbaity material is common at Medium. There are a lot of self-help guides too.
That (understandably) puts off certain writers, such as Rupert.