why diabetic shouldn't eat mango

I'm not sure who told you that mangoes are off limits??  Mangoes actually have a low glycemic index (51), meaning they enter your blood stream and cause a slower and steadier blood sugar rise when compared to other sweets or carbs.  In general, the new approach to diabetes and eating is to "count carbs" - so you would want to know how many carbs are in a specific portion size of any fruit, veggie or grain carbohydrate that you chose.  But off limits?? No fresh fruit is off limits - you just want to follow a specific guideline of how many carbs you are recommended daily and then most people with diabetes will fit in 2-4 servings of fruit daily within the carb count allowance.