Taxes are low and there would be a mass revolt if they were jacked up to European levels.
Fracking, and development of all that domestic shale oil that became economical to exploit when oil prices were through the roof previously, as I understand it.
Oil is sold on a global market. In fact the state of Texas is due to become a large producer/exporter than the nation of Iraq in the next year or two. Differences in price are due to regional taxes.