Why is impossible to save as from a version to another

why files made in Toon Boom animate PRO can’t be saved as Animate files or even toon boom 5 can’t save as to toon boom 4, or 3?
why you made your programs impossible to comunicate to another? why??

Because the new versions have new features. They are forward compatible but not back I think.

Exactly. Files made in the older version can be opened in the newer version, but not the other way around. New features are added, and file structures may be changed.