Fiat is the most convenient and efficient form of money. Bitcoin and other crypto currencies are nothing but a wet dream of people to replace fiat in the future. Right now they are horrible forms of money, and that is almost every aspect.
Another thing is that people don't want to spend something that appreciates in value, which applies to Bitcoin and gold. Fiat on the other hand is something we are incentivized to spend due to inflation.
There is no cloud without a silver lining
If people prefer to spend fiat, say, the dollar instead of Bitcoin, it basically means that they value the latter higher than the former. And there should be a reason for this. So we can't actually say that Bitcoin is a horrible form of money per se. I agree that it sucks as a currency while fiat and more specifically, the dollar, shines in this department. But if we don't consider a currency aspect of money as the only one or the only important one, Bitcoin may not be that bad after all (for example, as a value transfer vehicle)