An interesting difference between OUI and vehicular homicide is that one makes a decision to commit the former. Driving after drinking is something one chooses to do, not something that happens to the individual.
ANYONE can commit vehicular homicide without any element of intent, choice, mens rea or scienture. It is a true "there but for the grace of the FSM go I" kind of crime. Sunlight in the eyes, someone stepping out into traffic, a second's lapse in judgment, falling asleep at the wheel, medical episode that does not leave a confirming biological footprint (fainting as opposed to a heart attack and the like) can all result in a vehicular homicide charge and quite possibly conviction. The system wants to balance the scales and make someone pay.
Add in that the driver is unlikely to have the presence of mind to realize someone may have been killed, and understand the desire of the system to hold someone accountable, the driver may make some spontaneous utterances that are later used against him/her in trial - as in "You told the officer you nodded off, and now you expect us to believe you passed out from some medical episode?". The officer asking if you need an ambulance and looking out for your welfare is the same person tasked with gathering evidence used to prosecute you. It's a natural setup for the classic Reid-Inbau technique.
I was a passenger in a fatal car accident. It took weeks for the police to release the report to me since the DA's office had to determine if someone could be charged. And this was a case where the police had concluded that the driver who was killed was 100% at fault. Still, by procedure, a scavenger hunt looking for someone to prosecute.
Just remember Chief Wiggim's classic line
Back on topic - a fundamental problem is the MA system's systematic de-facto policy of the courts not recognizing the lifetime removal of a basic constitutional right as punishment.