The case didn’t come apart so much as it never came together. And the prosecutor’s closing statement that “Somebody in that house killed Caylee” was pretty clear in being an admission that they were not certain that Casey had killed the little girl. You can’t get a jury to go beyond reasonable doubt when the prosecution itself has reasonable doubts.
They demonstrated in that closing that the parents clearly were behaving consistently with a missing granddaughter, whereas Casey was out partying and lying to the family and police. There closing was quite powerful. Most thought it was a winner.
There was no accidental drowning, nobody puts duct tape over the nose and mouth of a drowning victim. It was a murder. Casey repeatedly lied to her family and police about where her daughter was. Casey clearly had a guilty knowledge not displayed by her parents and brother. Even if circumstantial, there was sufficient evidence for conviction.