PS. Look at OJ Simpson. Found not guilty in the criminal case. Still lost in the civil case.
What the government can and can not do is based on state law. In OJ’s case, since the criminal trial had already been held, he was not going to give anything up as a result of the civil case. In this case, apparently, the prosecutor thought that not prosecuting criminally would enhance the win probability of the civil case. He also reasoned he didn’t meet the standard required for a win in a criminal case. He was doing what he thought would be the best given that the civil case would have been weak.