Several reasons.
1. Kodak does not make acetate itself
2. Kodak does manufacture Estar
3. Acetate supply is sporadic at present and for the foreseeable.
4. Acetate prices are highly volatile at present and for the foreseeable.
For Kodak, it might be a case of "You want your pro films, have them on Estar or not at all". Leaving what acetate it can scrounge for the motion picture films if no substitute is possible.
Eastman Kodak buys Estar branded polyester virgin pellets from Eastman Chemical in Tennessee.
Eastman Kodak extrudes Estar in Rochester.
Polyester is in demand, Eastman Kodak is a very minor player for Eastman Chemical compared to packaging and fiber.
Considering that there's no where near enough volume for Eastman Kodak to extrude both materials, and projection films require Estar thus conversion to Estar for amateur films.
Cine camera film requires acetate, polyester can destroy a camera in a jam.
I suspect EK is simply trying to balance out production, and most importantly show whoever is extruding the acetate that EK has options.
Never forget cinema is Eastman Kodak's most important market, especially with renewed interest in film and EK doesn't have a single competitor in cine films.
MHOFWIW, I'm an idiot so what do I know. Having worked with plastic companies and with procurement departments leverage is sought by both parties. I suspect this is playing a role here.
Watch for Cine camera films, Double X, the Vision films. If these are converted to Estar, it could all go rather quickly.
Again I'm an idiot, I'm only guessing here.