First because overdosing on insulin is fatal. That's not unique to insulin; overdosing on Tylenol is fatal too, but with insulin you don't get any warning: With

First because overdosing on insulin is fatal. That's not unique to insulin; overdosing on Tylenol is fatal too, but with insulin you don't get any warning: With a slight OD you'll feel your brain get foggy and you can chew some candy to fix the problem, but with a big OD you just pass out and die. Second because the peculiarities of insurance often mean prescription drugs are cheaper to the patient than OTC drugs. Making insulin OTC might well make it more expensive.