That's a good question. I think there are three reasons. First is the huge increase in supply from shale oil in the United States, which basically makes the U.S. the biggest producer of oil these days. Second is the much slower growth in demand for oil because the global economy is slowing. Third is the politics of the OPEC countries where Saudi Arabia is refusing to reduce its production of oil.
On October 28th, 2014. See this statement in context.