Manslaughter is about proving negligence or misconduct. The prosecution case was that Baldwin was at fault as he was negligent handling a gun with live ammunition.
Part of Baldwins defence was that he did not know the gun had a live round in it.
The new evidence was that the live ammo came from the props company, not the armourer, throwing doubt over whether the armourer or Baldwin knew there were live rounds on set or in the gun.
That’s a hugely important part of the defence case, and also makes it much hard to prove involuntary manslaughter - it would be negligent to fire a gun knowing there is a live round in it, but if you did not know there were live rounds then does that meet the same level of negligence?
Personally I thought the case against Baldwin seemed tenuous so I’m not surprised this new evidence ended the trial.
This does raise serious questions about the safety of the armourers conviction. She might still be negligent as its unclear how live ammo from the prop company got on set without her knowing but she has not been able to answer that as the evidence was suppressed and she was convicted on the assumption it was entirely her fault the live ammo was on set.
It raises even more serious questions about the behaviour and motivations of the new mexico prosecution team and investigators.