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).
Btc has eliminated having to deal with middlemen and third party sponsors which leads to not anonymity and security between traders also it's much more cheaper in the long run when traded rightly. Once more vendors start accepting btc or crypto as a payment system it will be a very good payment system.