The answer is that it often is not, at least for most people, most of the time. Eventually it will be the best payment system, but it needs to develop further, gain more adoption, and get over the scaling obstacles. Even so, Bitcoin is already the best payment system for certain use-cases, such as sending money across borders, or receiving money from someone you don't trust (which helps further in cross-border transactions).
As of now, bitcoins aren't the most efficient way of paying your bills and sending money to people. You might want to just stick with fiat money transfer today until the technology for bitcoins have become better than now. But I still do believe that one day bitcoins will be as much or even more effective than fiat transfer.