Thank you for your comment! Publications do have a wide audience, but an article that is not distributed in topics but appears in a publication will only be found if a user goes to that publication’s homepage — and a casual user of Medium is probably not navigating to the homepages of all the publications they follow.
A notable exception seems to be The Writing Cooperative; many articles here are about Medium itself and therefore ineligible for payment in the Partner Program… and yet, they tend to get quite a bit of organic traffic. Just speculation at this point, but it might be a publication that writers intentionally navigate to in order to read the latest posts. I’ll do a more thorough analysis after I’ve written more members-only articles for them.