Often times, when a mistrial is declared and a retrial occurs, prosecutors will change strategy. Additionally, rulings from the previous trial are not automatically carried over, so suppressed evidence (for example) can be potentially displayed during a trial if the judge rules differently.

Another factor, which is more common in state courts, is the lesser charge consideration. Upon a retrial, the judge can instruct the jury to find a defendant guilty of a lesser crime in lieu of the originally charged crime. For example, manslaughter instead of murder.

Overall it’s obviously stacked against the defendant in federal court. Adding in unlimited retrials basically guarantees a defendant will be found guilty eventually.