Because everyone stopped driving.
Not really, of course, but the trends against driving have gained the upper hand in recent months, which has caused the demand for oil (and gasoline) to drop. Several suggested readings:
In my original response, I linked to an FHA graph showing the 12-month moving average since 1983 to make clear the drop in the number of vehicle miles traveled within the US. The original FHA graph is rather difficult to read, so I've made a new graph, below, showing the same data:
If it makes you feel any better, The Economist is reporting that oil prices may be going back up to around $150 or so by 2010.