Substack thrives because it's built a reputation for having well-reasoned or engaging voices in a newsletter format. They pushed a big campaign to onboard writers like Bill Bishop (Sinocism) onto the platform early on. Because of that, newer writers are likely seeking to tap into the network effects/discovery systems that comes with having big names on the platform.
People log onto Instagram, Tik Tok, X, etc. because the people they're interested are already on the platform, and the platform provides a recommendation system that allows newer "creators" to tap into an existing audience for others in their niche.
Did a short talk on this at a local meetup recently, basically on how we might collectively re-create the entire stack of what Substack offers, in an open ecosystem, with a strong focus on network / distribution, more so than just features.
It's a tall order but I do think there's a big opportunity here beyond what any single app can or should tackle, in building an open alternative to the whole longform publishing stack, combining the tools and the social network side. AT Protocol and the Atmosphere (ecosystem of folks building on it) are pretty well positioned here. Lots of nice ways to get autonomy and ownership with web publishing + distribution and social affordances at the same time.
Of course a big challenge in bootstrapping the network and building the cultural footprint and giving folks flexible ways to make money and reach new readers, but I'm encouraged by how much progress we've made collectively just this past year.