Just because something is shot on film doesn't mean the video files produced have been mastered for UHD/HDR. That's a level of processing that still has to happen before it reaches the consumer.
If a film has already been mastered for DVD, it has to be re-done for Blu-Ray, and re-done again for UHD/HDR.
Not all films are popular enough to justify that repeated processing. "Ah, DVD looks fine..."