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).
It has got its own advantages and limitations and lot of improvements need to be done. For instance, cutting down the transaction fee for slower transactions is required.