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).
Consider Bitcoin as the best payment method is still questioned because the disadvantages that this practice brings are greater than its advantages.
Only on specific occasions does bitcoin act in such a way, and between it, we find when the transactions are international since it facilitates the immediate ones of the same one.