[Haskell-cafe] Why isn't ArrowChoice a parent of of ArrowApply?

In Control.Arrow we have: |leftApp ::ArrowApply a => a b c -> a (Either b d) (Either c d)| Any instance of |ArrowApply| can be made into an instance of |ArrowChoice| by defining |left = leftApp|. So why isn't |ArrowChoice| a parent of |ArrowApply|? Best regards, Petr