Most people do not like to take risks with their money. For many people, the government is the safest organization they can trust on. So the money issued by central banks is the safest for those people. Central Banks regulate the money supply and try to boost the economy with monetary policies. However, Bitcoin does not give them so much reliance in comparison with fiat money. They consider bitcoin as a volatile, dangerous medium of exchange. In that manner, there is a difference in BTC and fiat currencies.
I would say a little differently, the absolute majority of people do not like to risk most of their money and have their own reasons and every right to do so. For all their shortcomings, the fiat currencies of central banks are one of the tools of public administration, and no state will voluntarily give up such an instrument of influence on the economy and on other spheres of life. Therefore, it is a big mistake to contrast fiat currencies and cryptocurrencies.