CHE didn't want visitors. They wanted subscribers. The Fora didn't seem to provide subscribers. The effort it took to keep The Fora going wasn't worth it to them.
The way you get subscribers is getting visitors, who are then exposed to the subscriber-only content.
The way you keep subscribers is building personal engagement and connection on a website, which keeps them returning and remembering why they didn't mind paying for content.
Maybe so. But it's hard time for journalism and there's some liability in having random people add comments on pages that a web based source hosts. IHE also scrapped reader comments. I remember there was once a thread on the old fora about how many people there had CHE subscriptions, and almost no one did. So it didn't work for them. Maybe if they had done it differently, it would have. As I recall, they didn't even run ads on the old fora. I haven't seen any other online journalistic publications which allow completely free range reader generated discussion and use it as a way to generate subscriptions.
NY Times and other papers do continue to have reader comments on articles. Slate does it too. I'm guessing that they hire one or more people to supervise those comments.
These days Reddit occupies the main space for free range discussion. That relies a great deal on volunteers. There's Twitter, and academic twitter was interesting. It probably still is but I left twitter.