because peeps have long history of knowing how much things are in fiat, e.g. everyone knows approximate price of loaf of bread in fiat, so say its $1 where you are, the price will probably was $1 last week and will be $1 next week more or less by few cents. Peeps don't know how many btc for a loaf and they certainly don't want to know that it was 0.005btc last week but today is 0.01btc and who knows how many btw for a loaf the morrow?