Because the shooting is still being actively investigated, both as a police matter and as a potential civil rights case.
Why didn't the kid just drop the gun when he was ordered to do so by police?
That’s a good question. Another good question is, why didn’t he drop it when Ismael Mondragon told him “throw that thing away, the cops are right behind you”?
There may not be any dashcam in that car, or if there is, the confrontation could have taken place off to one side of its field of view.
A big question might be:
Was that the order they gave him or was it "turn around, put your hands up and drop the gun."
Apparently he did turn around and might have been in the process of complying with the rest and as he was raising his hands over his head the barrel of the gun inadvertently pointed in the direction of the police and that was all they needed to say they felt threatened and fill him full of lead.
Why didn't the police give him more time to comply with whatever their order was???
“Why didn’t the kid just drop the gun when he was ordered to do so by police?”
It wasn’t a gun. Why don’t you get that? So when ordered from behind to drop the gun or the weapon a 13 year old might well have not know what the cop was talking about or even who the cop was yelling at. He might have even turned around to see who was yelling.