PVDF is expensive (in electronics, the best known brand is Kynar). $15/lb for the raw resin. The PEs are more like $1/lb. A bit more for XPE. Now you can figure out what you're paying in markup.
OK, but some plastics are easier to make in thin films. PET is one. That's why, for a given rating, PET caps are much smaller than most of the film alternatives. It's also strong, making it easier to wind. Good to pretty high temperatures, so easier to coat via plasma vapor deposition.
Sheldon