When the prosecutor failed to prove cause of death that left the door wide open for the defense to interject reasonable doubt. Most of the prosecutions case was based on circumstantial evidence and that's what cost them. The prosecution went to trail too early based on public sentiment and failed to do enough good old detective work and come up with some actual, physcial evidence to tie her to her childs death.
Personally I think the mom was involved in the childs death in some way but failure to prove cause of death made it difficult to directly tie her to anything without the defense being able to interject reasonable doubt. The defense did a helluva job. I'd want that guy as my legal eagle if I were ever toface major charges.