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).
Currently we can not use Bitcoin as a payment, because it is impossible to set up a payment system with a price changing between 7000-10000 USD for 1 day. So, think about it, you go to the store with 0.01 Bitcoin, but at the end of the shopping money is not enough and the prices are changing. I mean, it's a little hard to do this now. When Bitcoin's price will be stable, then maybe it can be used.