Because of what ethanol does to the tanks:
imagine what it'll do to your car's fuel system.
Almost all underground fuel storage tanks now are made of fiberglas and are impervious to to corrosion, the problem is the water itself and collected residue from the venting system; if gasoline is added to a tank that already has too much water at the bottom there is a great risk of phase separation every time the tank is filled until it reaches a point where the raw water and alcohol will be pumped directly into the gas tanks of cars.
Proper station maintenance requires periodic pumping of the condensed, collected water which will be contaminated with some of the gasoline product when pulled out of the ground and disposal is quite expensive since it requires the use of special overpack drums and hazardous waste final disposal methods.
Station operators who don't perform this maintenance are beseiged with complaints and often find themselves answering to the pollution police.