Its entirely possible they stayed with Chiltern for so long as they didn't by then have enough Chiltern-spec Mk3 coaches to replace them with. Image consistency, especially when it comes to running mixed-livery rakes of coaches, is very important to some operators.
Don't mistake a lack of repaint with a lack of desire to repaint.
Another reason to consider with GW not repainting a full rake of trailers into a "heritage" livery, is availability to release yet another trailer set for repaint. With the existing commitments to repaint sets (ROSCO obligation, be it blue or green) seeing full sets away at Kilmarnock for weeks at a time, and decreasing reliability thanks to age, can GW afford to release another set of trailer cars, which for the brand reasons above will also have to be kept together or run short, on top of the 2 green sets that already can't mix with the rest of the fleet.
I'm not privy to the decision making process, but knowing a number of people who would have been involved I'd be surprised if willingness was a factor!