Except that it requires a minimum of tech-aptitude, and also a less smooth workflow. Post somewhere, then prepare an email for Mailchimp or the like, then send it, track it afterwards, etc - all of these are complexities for the masses.

For tech people or tech-savvy creators, Substack doesnt provide much. But for non-technical people, its the easiest way to start a blog, have a mailing list, a subscription site at the same time. Moreover, it has easy statistics and all that. And its extremely simple to use.

That's why it will likely be even more popular in the long run.