There are many reasons and speculations but I can name some here and people can add their own opinions/comments as needed.
- It is the simple economics of supply and demand (e.g. China's demand has been falling).
- United States domestic production has nearly doubled over the last several years.
- Inventory is at record high.
- Saudi and many nations' oil supply that was sold to the United States is competing for Asian markets, and the producers are forced to drop prices.
- Non-OPEC oil production and exports are rising year after year including the Russians.
- Unwillingness to cut production to allow prices go up because producers afraid they will lose market share and benefit their competitors.
- Iranian oil exports are coming back into the market.