While it would be a great thing, you have to remember how long it would take for the game companies in which most of the profits go to (sans the royalty Valve gets) to agree to a decision like this, as money could possibly get to them a lot slower than with Paypal, etc. Also I feel like there could be customer service issues since all payment methods that deal with Steam have billing addresses, and while some BTC wallets programs do (IIRC? I don't deal with BTC much so), I feel like the anonymity with some sites could get in the way of refunds, community market purchases/selling, etc. Lastly, an implementation like this could take a very long time, and isn't as simple as you make it seem. Would it be great? Yeah. Is it feasible? Yeah. Is it easy to implement, a reliable enough currency, and will there be almost no customer service issues? Probably not, which is the issue.