Why do diabetics get glycosuria?

Renal Overload. Glycosuria (or glucose in the urine) is caused by the kidneys(proximal renal tubules) being unable to reabsorb all the filtered glucose. Diabetes due to either Insulin resistance or lack of Insulin results in underutilization of glucose by your cells and hyperglycemia. Glycosuria usually occurs when blood glucose level exceeds 180 mg/dl.
Kidney normally ... Reabsorbs blood sugar filtered by kidney but when sugar over ~220, blood sugar 'leaks out ' into urine- recognized as 'glycosuria'.