Why would the U.S. government tolerate universal currency when it has something far more advantageous? A currency it controls every other currency is pegged to (including Bitcoin).
More practically speaking, the government needs the ability to expand the money supply during financial crises. Bitcoin doesn't lend itself to any of those Keynesian tricks.
And, government is not "afraid" of Bitcoin. Letting the experiment continue is more about laissez-faire capitalism than an inability to regulate crypto.