There wasn't enough evidence against her. I'm looking at what was presented and it seems like they pulled out whatever they could, half of so being bs. In my opinion, I think she didn't kill her but rather freaked out and tried to hide her dead child. She in my opinion still deserves jail time, 1-5 years, but nothing more.
Events like this happen all the time, what made it so big was the death penalty was brought up. You have to understand, this happens a lot every day. (The more days that passed with a case, would have helped bring about the ability to gather more evidence).