Why didn't Giorno heal Polnareff's body and restore his soul to the body?
It's not addressed canonically as far as I'm aware but I can think of at least two possible reasons:


It's too risky: Giorno was able to heal his own body and transfer his consciousness back because he used his own stand to do so. Either Gold Experience Requiem can't transfer a second party's soul between bodies or they aren't willing to risk the possibility that Polnareff's soul would vanish in the attempt
It's a temporary solution: As we saw with Bucciarati, Gold Experience can't completely reverse death, just temporarily bind the soul to the body as it decays. Given the choice between this and staying a turtle, Polnareff may have chosen the later.