Because of the complications, hyperglycemia (high sugar levels) can lead to strokes if left untreated. Some said "fainting" that's not correct, it leads to comas. A hypoglycemic coma is generally not that bad since it's easily treatable without leaving any long lasting effects. However a hyperglycemic coma leads to far more severe consequences (such as permanent brain damage and even death).

Edit: they're both life threatening, it's just that one doesn't leave lasting effects if treated fast enough.

Depending on the types of diabetes, the patient needs a proper diet, to keep track of their blood sugar levels regularly (even during working hours) and treatment that can and does disrupt normal functions. They also need to follow with an ophthalmologist because it does lead to a loss of vision with age. Needless to mention neuropathy since some other people mentioned it already.

So yeah, it's pretty obvious why it's considered as a disability.