Well, because fiat currency has been around for centuries and has built up enough trust in that time to be used as a reasonably stable exchange of value.
In my opinion, it hasn't. It's just that current monetary system is purposely difficult to understand for that very reason; for the masses to accept it without even thinking how to change it.
In many countries fiat currency might be the money used but a business can choose to accept whatever payment method they choose and obviously they go for the ones most convenient to the customer.
They're forced to use the fiat currencies. You can't state that you'll be accepting only bitcoin when there are laws that say otherwise. You may provide it as an alternative payment method, but the merchant has to then convert them to fiat, to get taxed.
Also, it's not as obvious as it sounds if the customer is unaware of what's more convenient to them.
A major obstacle for wider scale adoption on everyday transactions will be, in the current form of Bitcoin, the fact that you have to wait a variable amount of time for the blockchain to confirm enough for the business to be happy that you're not going to double spend and leave them without payment.
Lightning Network.