“The Bitcoin protocol follows a declining block reward schedule, whereby the number of bitcoins awarded for confirmed blocks halves every 210,000 blocks. The halving of the block reward is a significant negative shock to miners’ revenue, similar to a large discrete price decrease.”

http://libertystreeteconomics.newyorkfed.org/2015/08/entry-and-exit-leads-to-zero-profit-for-bitcoin-miners.html#.VeG3130oef4