In fact they kinda have to in the Italian System, don't they? They have to write a Motivations Report summarizing all the evidence against the accused. Then it goes automatically to the appeals court, which then reads it along with the defense appeal documents. This happens for every single crime of this nature which isn't fast-tracked, and as I'm sure you heard, a lot of them get overturned.
In this case, they relied on a lot of dubious 'evidence,' which Giancarlo Massei had to spend pages and pages trying to establish as legitimate and relevant. I'm betting he didn't want to do that, in fact he might have envied Michaeli getting to write Rudy Guede's Motivations, I don't think they were paid by the page. So if Massei has to continuously accept low probability scenarios, and even those which defy known scientific principles, it's probably a pretty good hint the wrong verdict was reached. He might even know that as he's writing it, you can kinda tell the way he just piles bilge on top of the cesspool which is what the 11:40 time of death argument amounts to.
Hellmann is not bound to any of these arguments. He's free to dismiss the luminol stains as meaningless, which is what they very probably are, being as they tested negative for blood and a fair number resemble abstract art rather than the foot or shoe prints they're supposed to be. The judge in the trial of the first instance is bound to try to make a way for the evidence to work if a verdict is reached, he can't just throw up his hands, tell the world 'I can't write a Motivations Report using this evidence! Sorry, I didn't realize how silly the case is! Let 'em go!' Instead he tries to think of a way it could still possibly work, which is what he did all the way through the report on every single piece of physical 'evidence.'
Perhaps that's the part that's confusing you, the silly theories in Massei are just indications of how poor the verdict is and how the evidence doesn't support the conviction. They're not set in stone, and where they conflict with observed scientific principles, logic, and probability, Hellmann is highly likely to choose the more rational explanation. Massei couldn't, he was bound by the verdict and had to try to make it work, Hellmann gets to see how silly it was and not make the same mistake twice!