Okay guys, finding a realistic answer is better than just stamping it off as a KASU, right? Plus, that scene was recreated at TAC, so it couldn't possibly be a KASU. I mean, recreating the error is just wrong.
The simple answer might be that he was too weak to morph. By the way, Mertil was somehow incapable of getting the morphing power, i think.
Morphing needs energy, and Elfangor probably worn all of it. He must be pained to death by that time.