This is because the biceps is relatively weaker in this wrist position and thereby forces mores stimulation of brachialis muscle. (exercisegoals.com)
A good arms workout focuses on hitting all three heads of the triceps, the biceps brachii and brachialis as well as the forearms. (fitnesscrest.com)
... HOWEVER, in you case (assuming your biceps brachii is fully detached distally), ANY bicep curl exercise will work brachialis about the same as the preacher curls or concentration curls — since the biceps brachii is not involved in either scenario for you. (kingofthegym.com)