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).
I somehow doubt this but bitcoins are far from over if we talk about its growth. Bitcoins have a lot of updates to go through so we can expect that bitcoins will be an immediate replacement to our payment systems. As of now, I'll be sticking to fiat transfer because of cheap and fast it is to use.