The problem is simple: all those “boutique” gasoline blends. That means you can’t easily transfer refined gasoline to anywhere in the country, which drives up the price of gasoline at the wholesale/retail level.
Boutique blends are probably more of an issue in the summer months, too. You see it here in New Jersey and Pennsylvania. Fuel taxes are much higher in PA, and this is reflected in a price difference of 30-40 cents per gallon in the winter months. But NJ has special blend requirements for the summer months, so at this time of year there usually isn’t a huge difference in gasoline prices between the two states.