It didn't fail, people are using Bitcoin as a currency, maybe just not at a scale that some people would like to see.
Also, if you are referring to the fact that it's main use case is investment/speculation vehicle, then it can easily be explained: Bitcoin is still very new, and only recently started going really mainstream, so there's still a lot of uncertainty, whether it is going to work or not, so this causes volatility and discourages big companies from accepting it (and even those companies that claim to accept it now, like Microsoft, are actually using proxies), so we don't see any global adoption yet. But it's going to come sooner or later, when public will get more confident.