Because the price is set by the last 1%.
If we can get 99% of our oil out of the ground for $40 per barrel and 1% of our oil out of the ground for $100 per barrel- then every barrel sells is if it cost $100 per barrel to get out of the ground.
And that's just in the united states. Europe also has a similar size fleet of electric vehicles.
And in Europe, for instance, while total petroleum consumption averaged over 15.3 million barrels per day in 2009, it was under 14.3 million in 2013, and has dropped further since.
We get 19 gallons of gasoline per barrel so that's so 465,000 fewer gallons of oil here (and another 465,000 fewer gallons of oil in europe) translates to 48,000 barrels a day of oil that used to be needed that isn't needed any more.