DUI is special. Prosecutors aren’t supposed to drop charges. This is a legal rule in addition to a political rule. Many states decided that it is better to put innocent people on trial than risk having prosecutors set guilty people free. There are procedural rules for DUI cases that don’t exist in murder cases. In Massachusetts the prosecutor is not allowed to drop a charge of repeat offense DUI unless he proves in court that he can’t get a conviction.
Many DUI defendants have to face trial despite flaws in the case. (Not most, but many.) They have a choice: judge or jury. When the facts are clearly on your side you want a judge.