Have you been stockpiling bitcoin to go on a Steam shopping spree? You'll need to change your plans. Valve has stopped accepting bitcoin due to a combination of high transaction costs (up from 20 cents in the beginning to $20) and "volatility" in the cryptocurrency's value. While the virtual cash has never been especially stable, its worth has taken a roller coaster ride over the past few months. Its value has been skyrocketing lately (one bitcoin is worth $13,300 US as of this writing), but it has also crashed hard -- Valve cited a 25 percent drop "over a period of days" as an example. That fluctuation creates a huge problem if you make a purchase and it doesn't complete before the usual bitcoin price guarantee elapses, as you may have to pay an outstanding balance and a second transaction fee.
Source: Steam Blog