I think US still afraid of cryptocurrency domination with its own currency the US dollar. if the facebook's cryptocurrency Libra will become successfully launched and use with its billions of facebook users worldwide im sure it was a really big development to watch that maybe can affect the US dollar superiority in the long run. that's why US regulators still doubting in the cryptocurrency development particularly the Facebook cryptocurrency project called Libra.
it would actually be a binding advantage if Facebook is subject to regulations where libra stable coins are required to use usd as a reserve currency of the same value, and each transaction uses a protocol from a consortium of trusted parties as happened to usdc. Facebook will always be subject to government regulation control and cannot run at will in a centralized system that can freeze them at any time.