Sadly, they can't charge JB with that because it would jeopardize any future cases against her. If she were to be found guilty of that (lesser) offense, no jury could find her guilty of murder if the DA ever felt able to bring those charges. The two are mutually exclusive.
She couldn't have left Sky alone in the car AND murdered him -- the two could not have possibly happened together. She either left him alone in the car and someone kidnapped him or he wandered off and succumbed to the elements (LE has pretty much ruled both of those options out) OR she murdered him/he died of neglect and abuse and she lied, saying she left him the car, as a cover-up. It simply can't be both. And, I think we all know which one it is.
So, if LE ever got enough evidence to charge her with murder by means of neglect or abuse, they would be screwed if they had previously charged and convicted her of leaving him unattended in the car -- because he would have never been in the car to begin with!
It's like accusing me of robbing a candy store, charging me with that crime, finding me guilty, and sentencing me to a year in prison....only to find out 1 year later I actually stole candy from a baby on a street corner and there was video footage showing that I committed that crime at the exact same time as the supposed candy store robbery I was previously found guilty of -- and had served my time for. You couldn't charge and sentence me again. It is only possible I did one of those crimes and it doesn't matter how late the proof comes in -- you can't punish me twice. That's simply how our legal system works, it's not fair, JB knows it, and she's getting away with murder, IMO.