Because of lack of investment and Hugo Chavez taking all the money from the oil companies to pay for his social programs the last en years. Chavez now has to resort to rationing gasoline. It is now cheaper to buy oil off of the world market at 100 dollars a barrel than to process there own heavy fuels. Add to this Venezuelans since of entitlements to cheap gasoline and the recent explosion at there biggest refinery and you have a recipe for disaster right before the elections.
All of this points to a serious problem brewing in Venezuela.