..Cause they can get it.
Do you have data to cite or are you just assuming that?
If true, on reason is could be that many states have laws that gas stations must charge a certain percent over what they paid for the gas. So if the price the stations pay goes down they cannot lower prices until they sell out the higher priced gas.
Competition makes the price go down. Anti-competition laws (basically anti Wal-Mart/Sams Club gas station laws) that keep prices higher would be to blame.