As somebody who has diabetic family members and has attended some lectures on the illness and its management: diabetes questions exhibit all the hallmarks of health questions which generate bad answers.
First, under ideal circumstances, people would know the current state of medical knowledge about diabetes management, and answer based on that. But the problem here is that there is no unified theory on what diet is suitable for diabetics. A generic "less sugar" is easy enough. But from then on, everything branches off. Should diabetics eat fructose? There is the theory that it is a great substitute for glucose, and the other theory which says that its metabolic pathway (triglycerides!) makes it more dangerous in the long term than the immediate effects of glucose on insulin production. Should diabetics reduce overall carbohydrates? Theory says yes, in reality it can lead to eating more fat, which again may or may not increase cardiovascular disease. And so on. There is no consensus on what diabetics should or should not eat.
Second, even though there is a handful of legit (but somewhat contradicting) theories about diabetic nutrition, most people simply don't know them. And when they do, they are likely to misunderstand and misapply them. They tend to doubt even some of the very basic facts, for example that, if you are using glycemic load, it matters a lot if you eat a cooked potato or first mash that potato and then eat it. This misunderstanding doesn't stop them from giving confidently-sounding answers based on hearsay, or from voting on answers which are written persuasively but contain huge flaws.
Third, although less concerning than the first two, such questions make rich breeding ground for crackpots. I think that our community would quickly shoot such answers down, should they appear, but it is a further irritant to be dealt with.
In short, I am against answering this type of question. A refusal to answer is unpleasant to the OP, but giving them convincingly sounding wrong advice is actually worse.
For the "how can we make them answerable" part, it is the same answer as for any other health question. If the OP already has a nutrition theory they like to follow, and ask us how to make the food comply to that theory using measurable criteria, that's OK. So, if the OP had asked "how do I reduce the total sum of starch and glucose in potato and carrot dumpling", that would have been OK. It might even have gotten the same answer as the now closed question. But the difference is that the OPs will have to know what their concrete goals are, as opposed to asking us to suggest something which is "good for them".