It's the air.
When different parts of the country have local air quality problems, they become an EPA non-attainment area. Then they institute oxygenated gasoline and reformulated gasoline (not the same) requirements. That means instead of one "type" of gasoline, oil companies have to make two types, but distribute all 130,000,000,000 gallons per year the US consumes through the original distribution system.
Now that MTBE is being banned, expect the distribution problem to go up some more. Ethanol as a replacement for MTBE... sounds good, but BZZZZT. Ethanol is so hydrophilic (it likes water) that pipelines won't let you send it through. That means the ethanol is shipped separately to the local terminal where it is mixed with gasoline (and where the truck picks it up from to deliver to the station). Worse yet, because the ethanol is so different from gasoline, the oil companies have to make yet another special version of gasoline just to be blended with ethanol... and now you still have to distribute it through the original system.
So shortages are likely, shipping and timing issues are more critical, and the distribution system is stresses because there are several types of gasoline out there. It will eventually be cheaper if the EPA requires reformulated gasoline across the country, because then the refineries and pipelines are back to dealing with a reasonable number of products.
Oh yeah; taxes, profits, supply vs. demand (anybody using less gasoline?) all are part of it, too.
