Why do patients with diabetes go blind, when there are so many more treatments they can have these days?

The light receiving/processing sensors of the eyes are fragile, needing a stable environment to maintain their function. Just like you would not survive well in an arid desert, they do not when their cells are exposed to blood sugar excesses over time. If a diabetic an keep their blood sugar in close to normal range, the damage is less. The same goes for certain nerve and kidney cells.

Lack of adequate control of blood sugar causes damage to the small blood vessels throughout the body, including the retina. There is conclusive evidence that controlling blood sugar prevents microvascular associated lesions like retinal damage/blindness, neuropathy and kidney failure. etc. Many patients with diabetes are non-compliant with treatment.