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).
To be honest, bitcoin is good currency if many country around the accept bitcoin as payment. Its more safe and we can carry around the world without have to exchange it to other currency. If many merchant can accept bitcoin, that will attracting tourist or people spend their bitcoin for shopping and that will increasing economic growth