That's because of the regulation that makes people not use bitcoin in their transactions. people are like "forced" by government regulations to only use bitcoin as their investment, while for regular transactions, they must still use fiat on it.

Also, people don't use bitcoin in their regular transactions because of fees. payments using fiat are without fees and waiting time, so it's faster and more convenient to use for regular transactions. but that doesn't mean that no one uses bitcoin for transactions, it's just that it's not often, like for international transactions or P2P.