On mobile so please take the suggestion to Google “prosecutorial discretion” as me honestly just trying to give you the info.

The basic concept is that a prosecutor/DA is supposed to be able to look at the full scope of the facts, and adjust the states response to a violation of the law. In this case that means that the DA is looking at the case and does not believe that there is a good case for arguing that the accused committed Murder 2. If that is the case, then is is both a waste of time and effort to bring a trial for that charge, and a an injustice to continue the trial.

To use a more absurd version of this, this is the same feature of the system that would be use if for instance, a cop randomly arrested the first person they found at the scene of a murder and accused them. It is intended as a filter for all sorts of reasons where we don’t want or need the full expense of a trial.

This is also happening on a broad scale all the time. I remember years ago cities started passing laws to deprioritize enforcement and prosecution of personal use quantities of weed.