Why do people use substack > medium?

Guesses:

1. Substack is popular. Consumers like to do what's trending for the sake of being trendy.
2. Medium has a proven history of treating its writers poorly. Almost every conversation I have with someone about Medium is a negative one.
3. Contrary to Medium, Substack seems to have a business model that's writer friendly. It's very explicit that by using Substack, you're starting a media business.

I've started using Substack for a weekly community building email I send out. I was using Mailchimp before, I've used it for years. But I wanted a change and something that felt more like publishing and focusing on content rather than creating and designing an email.

It's also free. I also like the fact that I could turn on charging for it at some point, if and when I feel I want to do that.

I think it has to do with two things:

1. newness (Medium feels like an established player vs. Substack being a new contender)

2. Substack is pitched as more of a newsletter/community platform, whereas Medium is more about writing and perhaps publishing to a less defined audience (newsletters are more en vogue at the moment)

I audited a bunch of different newsletter services and currently use Substack for MakerList. I use Substack because it's a newsletter service; the stuff you write gets directly sent to the people who subscribe to you. That's not the case with Medium, which acts more as a blog.

And I use Substack over other newsletter services because it's completely free and it's super easy to use. They only take money from you once/if you begin charging money at some point.

Substack seems way more writer-friendly; as a writer, you can decide what to charge and it's really clear what your share of income is. It's also dead simple to get started, and the UX/UI of substack is way easier than trying to spin something up on Ghost, or Wordpress.
Medium had some of these same things in the beginning (ease of use for writing), but their changes over the years (e.g., adding then removing custom domains, member program) have been unpredictable which is not ok for people trying to make money from their writing.

Like @csallen said, people use what's trendy, and as you see more and more people use something, it becomes a "standard".
For Open Startup List I'm using buttondown.email which is another simple newsletter service, probably more barebones than substack but it helped me send to 250 loyal subscribers the first interview of an open startup founder :)