I heard that it's because displays like this, which is an absolutely necessary procedure to dry the skin, is seen as extremely unhygienic for the hygiene rating test even if it's completely windowed in, because you're showing raw meat out in the open. So if a Chinese restaurant serves roast meat (which is basically every Chinese restaurant) it's borderline impossible to get a 5 unless they find some custom made equipment to substitute the drying method.

But those that have a rating of 1 probably just had a whole cockroach colony inside lol.