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).
More people would like to use crypto currency in that world and everyone will gain access to it. For now, it is a great payment system since there are a lot industries that accept bitcoin. In addition to that, I do believe that bitcoin has great future and great potential to become future currency. Lastly, I hope bitcoin will good enough in our near future.