Diesel has a much lower tax than gasoline per liter. The car manufacturers just serve the higher demand for diesel cars, because it is cheaper to operate monthly.
But try accelerating from every stop in the city all the way to the redline, pedal to the floor, but never going above 50km/h - you would never go above the speed limit, but the tank would disappear in no time at all. I'm certain that if I tried I could burn the entire 50L tank of the Polo in ~250km, by just keeping it in high revs and accelerating hard(as hard as you can in a 90bhp Polo anyway) and frequently. The injectors will happily keep feeding the engine more and more fuel as you do this, while diesel common rail systems have almost a hard limit of how much fuel they can inject - no matter what you do you won't go above certain values.
maybe in a frictionless vacuum!
At high speeds the force you have to exert to counter wind resistance is less than what you need to gently accelerate when going slowly.