Because some people need a stable asset to transact with for bills, and Bitcoin is not optimal for that. If I’m paying rent of $50 in a country on the verge of hyperinflation, then the volatility and fees of Bitcoin could turn my $50 into $40 when all is said and done. You’re going to lose a little less with stables.
Ultimately if you must fiat, then access to the “best” form is preferable.