Below the fold is an extended excerpt from this excellent essay by Kai Stinchcombe:
The original intended use of the blockchain was to power currencies like bitcoin — a way to store and exchange value much like any other currency. Visa and MasterCard were dinosaurs, everyone proclaimed, because there was now a costless, instant way to exchange value without the middleman taking a cut. A revolution in banking was just the start… governments, unable to issue currency by fiat anymore, would take a back seat as individual citizens transacted freely outside any national system.
It didn’t take long for that dream to fall apart. For one thing, there’s already a costless, instant way to exchange value without a middleman: cash. Bitcoins substitute for dollars, but Visa and MasterCard actually sit on top of dollar-based banking transactions, providing a set of value-added services like enabling banks to track fraud disputes, and verifying the identity of the buyer and seller. It turns out that for the person paying for a product, the key feature of a new payment system — think of PayPal in its early days — is the confidence that if the goods aren’t as described you’ll get your money back. And for the person accepting payment, basically the key feature is that their customer has it, and is willing to use it. Add in points, credit lines, and a free checked bag on any United flight and you have something that consumers choose and merchants accept. Nobody actually wants to pay with bitcoin, which is why it hasn’t taken off.
Plus, it’s not actually that good a payment system — Visa can handle sixty thousand transactions per second, while Bitcoin historically taps out at seven. There are technical modifications going on to improve Bitcoin’s efficiency, but as a starting point, you have something that’s about 0.01% as good at clearing transactions. (And, worth noting, for those seven transactions a second Bitcoin is already estimated to use 35 times as much energy as Visa. If you brought Bitcoin’s transaction volume up to Visa’s it would be using as much electricity as the rest of the world put together.)