My guess is that some of those ads bring in so much money that covering them with an ad-free model would lead to a cost that they feel is prohibitively expensiv

My guess is that some of those ads bring in so much money that covering them with an ad-free model would lead to a cost that they feel is prohibitively expensive. There may be ad agreements that preempt their ability to offer ad-free at this time, likely in the case of ultra-premium advertisers. It could also just be good ol’ internal bureaucracy at play. Maybe there was a turf war, and this was the best they could do while keeping everyone happy. Edit: The point in the other comment about it affecting reader demographics is a good one too.