Because the taxes didn't fall 75%. The cost to refine didn't fall 75%. The cost to distribute and market didn't fall 75%.
The price of gasoline doesn't rise as much as oil, and it doesn't fall as much either.
How much did you pay for a gallon of gas in 2011 when it was over $111 a barrel? I think the average was around $3.77 a gallon nation wide.
Since then the price of a barrel of oil has fallen to $40 a barrel.
Thus, the price of a barrel went down just shy of 75%. Why has the price per gallon not got down about 75%? By my estimate the price should be about $1.27 a gallon.
Because the taxes didn't fall 75%. The cost to refine didn't fall 75%. The cost to distribute and market didn't fall 75%.
The price of gasoline doesn't rise as much as oil, and it doesn't fall as much either.
About half the cost of gas/diesel is incurred after it is extracted.