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's still kinda new system for the people, even me, the bitcoin system of how it transacts. It is kinda risky tho because you can revive or refund what you've send bitcoins. And sometimes because it is anonymous, the users of bitcoin, it is hard to find where those people when you made a mistake and want to get back what you send.