This. Imagine thinking anyone in their right mind would prefer those currencies to dollars.
Also, Bitcoin isn't a market as big as the USD, yet, so trying to onboard the entire world to it, in a short time frame, would completely destroy everyone's economies and lead to a massive drain of wealth from the Bitcoin-poor to the Bitcoin-rich. It isn't possible to speed up Bitcoin printing, just because more people suddenly want it.
Also, pricing things in Bitcoin, would cause the prices of things poor people need -- LIKE FOOD -- to swing wildly and suddenly, causing havoc.