Bitcoin prices are so low, you see, that miners are spending more money running their supercomputers than they're making from new coins. So why are they still going? Well, they have dollar debts that they need to pay back, and where else are they going to get the money? They're stuck, in other words, in a catch-22: they can't afford to keep mining, but they can't afford to stop mining, either. (This, coincidentally, is the same dilemma that oil drillers who borrowed a lot during the boom face now during the bust). This has already forced one big mining group into default. And it's forced the rest to sell the only assets they have—Bitcoins—to pay back their dollar debts.

http://www.washingtonpost.com/blogs/wonkblog/wp/2015/01/14/bitcoin-is-revealed-a-ponzi-scheme-for-redistributing-wealth-from-one-libertarian-to-another/