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 I don't really think that bitcoin is a good system for now because there are still some flaws from it. Just few stores are accepting bitcoin and still not well-known to people. Bitcoin is still a stranger to people so it might be hard for them to understand its system.