Why? The role of a forensic psychiatrist is to look at all the evidence and answer the referral questions based on assessments, interviews and past research, not to take sides. A forensic psych who takes sides often finds that they've lost all credibility in the scientific community. The defence (who hired him) asked him to take a look at the likelihood that he was susceptible to suggestibility, so he did. He never said that he didn't/couldn't have killed the kid, just that in his view, based on information provided, assessments administered and a body of research into the area, it is likely that he's more suggestible and vulnerable to confessing to things than other people. Especially since US detectives use the Reid model of questioning suspects which is not best practice and is essentially manipulation, persuasive techniques and deception.
