USD is not only more liquid than bitcoin, it's more anonymous, too. One of the most important things about the Silk Road trial in my opinion was the fact that Ulbricht's lawyer claimed his bitcoin fortune was made through speculation. This caused the state to put on record for the first time that they can trace the chain of ownership of bitcoins to the court's satisfaction. And Ulbricht knew what he was doing, too - if anyone could anonymize BTC, I imagine it would be him.