So, assuming my recollection is correct and the price relationship between bulk rolls and cassettes did change, what in the way of Kodak bulk roll production changed to bring this about
If you know the answer please share it.
Sale volumes changed - to a huge extent. As did staffing levels and the sheer volume of operational machinery plus the resources to maintain it.
The per unit cost to make 100,000 is vey different than 1,000.
The film itself isn't more or less expensive of course. The cost of confectioning it is where the huge difference resides. That includes the "lost opportunity" costs involved in devoting limited resources that would otherwise be devoted to the highly in demand production of 35mm colour negative film in individual cassettes.
Eastman Kodak was at one time making 70 master rolls - that is millions of individual rolls - of Kodacolour each and every day. All of their machinery and systems were set up for those sorts of volumes.
The systems and machinery at Ilford has always been far, far smaller than that, and became far far smaller again when Harman bought the business out of receivership. That much smaller entity was and is better set up to do much smaller runs. In order for their volumes to get to the scale of the current, much downsized volume of Kodak colour film production, they would have to increase substantially their production capabilities - perhaps at the expense of bulk film capacity.