By offering a seamlessly integrated email, payment integration (Stripe is a big part of all these revolutions), and no-design-needed opinionated web presentation, Substack managed to basically resurrect a zombie category of text media that hadn’t really existed in this kind of pure form since the 90s.
Moreso, perhaps the user experience from Substack - the common look and feel - results in implicit cognition among users, something akin to "this is information that will help you learn something new."