They failed to prove both homocide and that she did it. The autopsy was botched. They never established that she died abnormally. Failing that, it was pure speculation.
I personally believe she killed her. However, certain proofs have to be met and the entire case hinged on emotion and speculation and absolutely no evidence. Every point the prosecutor brought out was refuted completely by the defense. They claimed 85 searches for cholorform now they determined that it was maybe once. Little things like that will destroy a prosecution.
How was the autopsy botched? All that was to be examined was bones sifted out of a drying swamp after the scavengers cleaned the bones. And that duct tape that kept the mandible affixed to the rest of the skull. The defense sure made no attempt to make sure an pristine autopsy was done, all they did was drag in some old guy that made claims about some imagined protocol about sawing a skull in two to examine the inside of the skull. A skull that had NO tissue left to examine.
Reasonable people would NEVER put duct tape around the head of their child having just died from an accident.