There is a third reason: It is pretty much unheard of for a prosecutor to let a grand jury see any evidence other than what is favorable for an indictment. For whatever reason, that was not what was done here. The grand jury naturally saw the conflicting evidence and declined to indict.
We all know why that was done here. Because the prosecutor didn’t believe there was evidence to convict.
In an ordinary case where the prosecutor doesn’t think there is evidence to convict, they drop the case. But he couldn’t do that here, because public opinion was so strong. So he figured he’d show it all to a grand jury.
In an ordinary usage of the grand jury, the prosecutor has already made the decision to indict, and the grand jury is just confirming it. Here, he was asking the grand jury to make that decision.
This wasn’t the ordinary usage of a grand jury. Hence they didn’t follow the ordinary procedures.
This is all good. Everyone says “a grand jury would indict a ham sandwich,” but we all agree that ham sandwiches should not be indicted. Rather, only people who the prosecutor believes there is evidence beyond reasonable doubt to convict should be indicted.
That is why not everyone in America is on trial–right? If prosecutors were supposed to indict everyone they could, and it is so easy to indict, then all of us should be on trial. But that isn’t how it works, because there is supposed to be someone deciding whether this person actually should be on trial.
Get it? Prosecutor decides who should be on trial, and (in some states) a grand jury then approves the decision based on some minimal standard. In this case, the prosecutor expanded their role to making the decision also.