Certain enzymes are chemically and allosterically arranged to break down certain proteins. If an organism has never encountered a certain protein before, enzymes it already has might by chance work sufficiently at metabolizing them. It's also likely that no enzyme they create will work at metabolizing the foreign proteins.
There are millions upon millions of different proteins - proteins are the foundation of life - the core product of genetic translation. To assume that are all exchangeable willy-nilly is asking for trouble, especially when it comes to dietary needs of captive specimens.
I only feed fish food which consist of products they would encounter naturally - if they are wild originating specimens. Farm bred fish are fine on most any number of artificially prepared mixtures, or natural alternatives.