Why does a mammalian kidney have such a large blood supply

Apparently there should be two reasons....

The large blood supply to your kidneys enables them to do the following tasks:

•Regulate the composition of your blood
•Keep the concentrations of various ions and other important substances constant
•Keep the volume of water in your body constant
•Remove wastes from your body (urea, ammonia, drugs, toxic substances)
•Keep the acid/base concentration of your blood constant
•Help regulate your blood pressure
•Stimulate the making of red blood cells
•Maintain your body's calcium levels

Basically, aside from all that info, we need the kidney to filter our blood, so it receives such a large volume so it can do its job, with the largest amount of blood possible.

not so much a blood supply as it is a blood capacity. the kidneys remove potentially fatal wastes from the blood. since not all of the blood circulated goes thru the kidneys on each pass, they do an extra good job on what does.