I see your point, but if Casey didn’t kill her, who did?
That wasn’t the issue at this trial. The prosecution can feel free to charge others they may think have killed her but they can’t charge Casey again because of double jeopardy.
My guess is that either Casey killed her or it was an accident and the coverup was to avoid charges of child neglect.
The operative word here is “guess”. There isn’t enough evidence to prove either, and the burden is entirely on the prosecution to prove the charges. They simply didn’t prove their case. Remember, she is innocent until proven guilty — our system is intentionally biased toward the presumption of innocence, and I wouldn’t have it any other way: I would never want to live under a system where I had to prove my innocence of a crime.