Even a cosmetic restoration would have cost an absolute fortune - with many external parts missing which would either need sourcing (from where?) or remanufacturing. I understand it's thoroughly stuck where it is because it has deteriorated and seized up, so just extracting the hulk to even start any work would also cost a significant amount of money.
Steamers are relatively easy to restore from total wrecks as there is nothing particularly technological about them - and the others you mention are unique prototypes that help tell the story. If the only thing in 15's favour is end doors (that would need reinstating anyway) then as others have said, there are related locos that show the concept, even if not the exact same model.