For some odd reason medium attracts the worst content. All those super optimistic shallow nonsense. Urgh.
Let me place my bet: Professional look (design, typography) + Medium brand + very little effort to publish + chance to get featured if it attracts clicks.
I think the lack of branding on Medium sites has something to do with it. People ended up lumping everything as "Medium articles", from spammy "growth-hacking" articles to well-researched, well written investigative article.