Probably because the GTT takes a couple of hours of the patient's time and occupies the lab, too. This is the only test that will tell the doctor if the patient can or cannot metabolize carbohydrates normally. I have always "passed" the A1c and have results as a non-diabetic. My last one was 5.6 last summer and is that good because of my low-carb diet and walking/exercises that I do. So, I can fool any fasting or A1c test and come out non-diabetic. When I took the GTT several years ago the doctor prescribed the blood sugar meter because of the high results. It's impossible for me to pass that test because I am insulin resistant - a diabetic that can control blood sugar with diet and exercise. The A1c can mark a trend of the blood sugar but it not reliable in diagnosing.
Laniediabetes moderatordiabetes type 2 controlled so far by diet and exercise
very low carb way of eating