Well..to the best of my knowledge,  I think that some plant proteins are inadequate for such replacement, considering their amino acid profile. Also , due to the anti nutritional factor (ANF) found in plants and ill thermal treatment in an effort to remove them, the proteins are dentured or fractions  of ANF may still be present in treated plants. Generally,  the plants found worthyfor such replacement are highly relished by humans for different purposes thereby creating an insufficient situatiins or poor access to the plants for livestock feed production. Finally , the necessary awareness has not be done such as educating the livestock stock cum fish farmers the need to harness this potential. Researchers  should not limit their findings only on paper publications rather the knowledge should trickle down to the rural and semi urban farmers who may be so much in need of it. Thanks....

In addition to what Stanley mentioned in his answer, fish meal has more phospholipid content and attractant components that plant protein that make feed with fish meal more palatable to shrimp. Thus you would need to relay more on other attractant when you remove fish meal.