It is the most basic of economic theories that the more supply the lower the cost. Due to the United States increasing production and Saudi Arabia not lowering production the total amount of oil available on the open market is higher than it has been as such being a commodity its price per barrel has lowered.
Bingo.
What most folks are missing is the logic behind what the Saudis are doing.
They can take a short term hit to the price per barrel, but what they're not willing to lose, is the American dependency on their oil. If they can keep the prices low for long enough, American fracking operations will begin to shut down, because it'll be cheaper to buy oil from the camel jockeys, than drill our own.