USD is used and is useful for transacting — especially in global markets — because of its high trading volume, stability, limited/no capital flow restrictions, federal reserve’s monetary policy to favor neither exporting or importing (meaning they will not chose to intentionally deflate or inflate the dollar’s value to favor/disfavor certain industries), and also USD’s global acceptance.
On the other hand, people don’t usually transact with BitCoin because of its volatility and because BitCoin’s currency deflation. I’m also almost certain Bitcoin protocols couldn’t keep up with global transactions even if the currency did stabilize. One last point (there are way too many to write them all here) is that there will be no incentive for people to verify blocks when all 21 billion coins are completely mined.
Bitcoin is a game of betting there is a bigger sucker who comes after you. Bitcoin whales realized that the government is the biggest sucker of them all. It’s ironic to me that they have lobbied so hard for the government to subsidize their assets when they have been preaching libertarian idolatry for years.
So do fiat currencies.