Greshams law applies here. If they make the good money illegal then it will increase the spread and drive a premium on the market for the good money. Then the cops which are being paid the bad money will be forced to accept bribes in the good money in order to stay on the job and feed their families because the state controlled money won't be able to keep up with their needs.

Then those who write the laws get their hands on the good money. Andreas Antonopoulos explains it well in this video: