I addressed this in the answer to Chris, but I think the secret is really that they’ve invested a lot in resources in a way other platforms have not, and those resources are probably useful to non-Substackers as well.

Ghost is a good platform. I don’t use it these days, but I’m a bit of a maverick—case in point, I used Ghost to build my newsletter before it had any newsletter functionality! (I currently use Craft CMS, which has a dedicated view that can spit out a completed newsletter template, with custom ads, in a matter of seconds.)