It's how they're taught to think about food and nutrition: People need a certain amount of calories to survive. People should not eat too much fat, especially people at risk for heart disease. But then they have to make up the difference in calories, so carbs must be the way to go. They also are taught that carbs are entirely necessary as an energy source for the body and the brain, and that the brain requires 150g of carbs per day to function. Sugar and white flour are bad, but other carbs can be very nutritious and good sources of fiber. Diabetics as a group are at higher risk of heart disease than the average population so they should minimize fat even more and thus need to make up even more of their calorie needs from carbs.
Some of that is true, some is not true (but still generally accepted - and therefore taught - as fact). A lot of it is not *exactly* true but has some basis in fact, only the studies those facts supposedly come from were never examined with a critical eye (for example, "diabetics as a group" presumes that all diabetics are exactly the same in terms of risk for complications...NOT!).
A lot of the conclusions medical and nutritional professionals arrive at are due to an extreme focus on heart disease. You've heard the phrase "media darling?" Heart disease is the medical version of that. Anything about heart disease will get a lot of attention in studies and in doctor's practices. When you are diabetic, often what gets the focus is not so much the diabetes as it is your potential risk for heart problems (in my experience anyway!). The actual diabetes care is not the star of the show. Hence why medical folks can both know, on the one hand, that carbs = sugar = increased BG, and turn right around and recommend a carb-based diet to a diabetic. Because the other option is to recommend a fat-and-protein-based diet, and that sets off all the much louder alarms in their heads about heart disease!
There is also the (incorrect) association between dietary fat and body fat, i.e. weight gain. And between weight and diabetes. The assumption is that weight makes diabetes and its complications worse, and weight makes heart disease risk worse. Or even causes both those things in the first place. So if heart disease is not the central focus in treating a diabetic, weight usually will be - but again, not the diabetes itself! Because diabetes is not (yet anyway) the sort of "medical media darling" that heart disease and obesity are. And once again, based on their training, they can't bring themselves to recommend a high-fat diet to someone who should be watching their weight AND their heart. So carbs it is.
It's not really their fault that they're taught it that way, nor is it their fault for learning it - they don't have any reason not to trust the education they're given and they don't have time to independently review every single study related to all the "facts" they are taught and decide for themselves. I do think it is a bit their fault once they start to see diabetic patients NOT succeeding in weight loss or BG control on their high-carb advice. Sadly, however, since medicine and nutrition are such complex fields, there is always some other option to consider besides jumping to "everything I was taught was wrong." Maybe this patient also had a thyroid problem. Maybe this patient wasn't being entirely honest about how much they ate. Maybe this person was eating the wrong KIND of carbs, or in the wrong combinations, or is taking other medications that are interfering, or... And these kinds of explanations are easier because they require less soul-searching and they don't risk you having to relearn everything you thought you knew, having to reevaluate the wisdom of teachers and colleagues you trusted, possibly being ostracized in your field for espousing a non-traditional view, etc.
I'm not saying it's excusable or acceptable, but it's not really a mystery once you understand the context they're working in.