[Subject 3] suffered severe damage to multiple limbs [In war? Terrorist attack?] several years before being selected as a candidate.
Unexpectedly to the goo aliens, even though the body he woke up in was able to shapeshift, the limbs that were lost early in [Subject 3]'s life were apparently impossible to manifest - ultimately leading to the conclusion the adult human mind was significantly hess adapable than they had hoped.
While there were continuted attempts to try and "rehabilitate" [Subject 3] to manifest these limbs, Basic human-designed prosthesis were produced in short order. [Subject 3a] has continued development of these prosthetics since.