I think from a technical perspective, newsletters are a solved problems with lots of options as you've mentioned.

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.