I’ve recently sold the site, so I can’t help much anymore. But I think it’s the combination of infinite scroll and the limited number of ad spots in the ad plugins.
With Vbulletin I knew all code pretty much by heart (as I know the infamous PHP/MySQL combination well). Discourse, being Ruby and Ember, appeared to be a too steep learning curve to get it right myself. I hope for OP (and others) that the team finds ways to increase revenue for everyone…