Why is inulin administration an effective way of measuring renal clearance rates?

Because it is not secreted and reabsorbed.

Inulin clearance is done by which the filtering capacity of the glomeruli is determined by measuring the rate at which inulin is cleared from the plasma. It is considered as the most accurate substance to use because it is inert polyssacharide that passes readily in the glomeruli without being reabsorbed in the tubules. Inulin is seldom used in clinical testing but used in research. Creatinine clearance is the more commonly used test to assess function of kidney clinically.