I don't see that as the case here, Sierra. New Hampshire has a sliding scale for DUI grades of crimes, from a Class B misdemeanor for a first offense up to felonies for Aggravated DUI, second and subsequent DUI, etc. There may not have been an applicable misdemeanor to charge, if this wasn't his first offense or if he was charged with some form of felony aggravated DUI because of the nature of the crash. In any event though, he was charged with DUI and not just the felony Homicide by Vehicle while DUI, so there were lesser included offenses. It wasn't the prosecutor that dropped the lesser included offenses in order to focus on the major felonies; it was the judge that dismissed those lesser charges.
If this had been an issue of evidence mishandling, or Miranda, then those issues would have been decided during whatever pretrial proceedings were held. The prosecution would have known going into trial whether of not they were going to be able to use the defendant's admissions, the toxicology results, etc. Now, I don't know New Hampshire's rules of criminal procedure, but a judge where I'm from would never entertain any motions to suppress that were made when a trial actually started and a jury had been seated. And this wasn't even an issue where the judge ordered that particular items of evidence be suppressed; he simply dismissed the charges.
This might be normal in New Hampshire, and there are reasons for it, but from my experience at criminal trials, a judge actually dismissing charges like in this case (and not simply making a ruling based on whatever their Exclusionary Rule is) is not something I ever encountered.