In my opinion, it is because bitcoin is still not accepted worldwide while fiat is official currency
Tim Draper, who is one the best entrepreneur of the current era and has his money invested in companies like Tesla and Twitter was of the view that fiat will completely vanish from the economy and this was 2014. According to him, in five years, people will laugh at the name of fiat because cryptocurrencies will be very prevalent and would be used all over the world.
Today after 4 years and 9 months of that interview, still a big portion of the world's population uses fiat which means that the market responds differently than the way they speculate it will. This is the reason why Bitcoin is still inferior to fiat because the costumers of fiat are more than the people who invest in cryptocurrency.