>>935786>supply and demandThis. Demand for oil was stagnant for quite some time before the price started to drop. Developed economies were not expanding as fast as they once were and it took a number of years for that reality to finally hit home in China, especially given the Chinese governments program of "hand out free money to build shit that does nothing". So, when the Chinese had to knock that shit off (a clamp down on lending, both official and shadowbank) and started dealing with the truly insane levels of corruption in their economy (difficult when, at least internally, corruption *is* the economy) the reality of "we don't need to buy oil, because developed economies aren't buying what we're making with it" exerted itself.
Commodities are down across the board. Oil is a leading example because the previous high prices encouraged alternative sources to go online, and now that they are the only way to pay off these investments is to pump as fast as possible. Inventories are climbing again and the cost of longer term storage will only cause more fire sales (heh) lest all the profit be eaten up, espeically combined with falling prices (which are also caused by fire sales).
TL;DR: Global demand is low, and previous prices encouraged an oversupply.