Ok, let's try it again... What was the reason to move all other film used in the industry (print, duplicate...) to PET but leave all original camera negative (and positive) films that would be shot in expensive cameras on triacetate base?
Well, there was a time when PET was not yet a consumable plastic as today. Due to its high price and difficulty in coating, it, it first was used at applications were dimensional stability was most important. That was not the case with the movie industry where this was no issue respectively where one could handle it.
You put up a good question though, but keep in mind that the movie industry is utmost conservative and had clinged to actually outdated materials in other aspects too, to an extend that I stopped questioning it.