The main reason they offer Diabetics the bypass rather than the band is because the RNY bypasses the bit of the small intestine that is closest to the stomach, so that area no longer receives nutrients.
My surgeon told me that this is the area where the insulin blockers are produced that lead to type II diabetes.
Attaching the lower small intestine to the stomach greatly increases the amount of glucose that is produced in his area, they reckon it almost doubles the amount of glucose that is produced, and this reduces insulin sensitivity.
In short the bypass re-routes our digestive tract and this alters and improves the way in which we control blood sugar.
The band helps by causing weight loss, increased exercise, and by reducing the amount of food that the body has to digest and absorb at any one time, but the results are most often less than for the bypass and much slower.
I was type II diabetic and took the maximum dosage of Metformine tablets and Exenitide injections twice a day. From the day after my operation I have taken no diabetic medication whatsoever