Murder one was not the only possible charge.
She was acquitted of all the charges that had her directly and deliberately harming the kid. Because there wasn’t evidence of that the things that happened to the kid were done BY her. That was the big problem with the case. There’s lots of evidence that somebody did bad things to the kid, there’s lots of evidence that Casey hid evidence, which is good for an inference that she’s probably the one who did the bad things. But “inference” and “probably” are not the kind of words that rise beyond a reasonable doubt.