For one thing, sheet film is on a completely different more expensive base from roll film. Then for something odd like 11x14 the entire cutting and packing sequence has to be stopped and recalibrated. No different than any other kind of material. Just try buying plywood in an odd dimension. Then you've got about zero competition in this category. And ironically, film prices have
been kept artificially low in the US for a long time. And then you've got raw materials, esp petrochemicals, now skyrocketing, so everything depends on the time of mfg.