The preponderance of physical evidence was circumstantial, but the actions of the defendant were anything but circumstantial. Caylee's remains were found after spending more than three years in a swamp, making forensic evidence extremely difficult to recover. Follow this timeline:
June 18, 2008 - Caylee last seen alive leaving her grandparent's home with her mother, Casey. Casey borrows a shovel from neighbor Brian Burner, only to return it about an hour later.
June 20, 2008 - Casey photographed at the Fusion, in Orlando, nightclubbing and participating in a "hard Body" contest.
June 30, 2008 - Casey captured on surveillance footage shopping at J.C. Penney and Target.
(Apparently, Casey had little concern that her toddler daughter had "gone missing" - or worse - inasmuch as she had failed to file a missing child report with the Orlando police.)
July 15, 2008 - Casey tells her mother and brother that she hasn't seen Caylee in a month, and that an imaginary babysitter named Zanaida Fernandez Gonzalez (Zanny) kidnapped the child.
(Missing for a month? And STILL not reported to a law enforcement authority? Probably because she already knew what had happened to little Caylee.)
July 16, 2008 - Casey arrested on charges of child neglect and criminal obstruction.
August 11, 12 and 13 - Meter reader Roy Kronk calls the Orange County Sheriff's Office to report a suspicious-looking bag in a swampy area. On August 13th the area was cleared as a place of interest.
October 14, 2008 - Casey arrested on charges of first-degree murder, aggravated child abuse, aggravated manslaughter, and 4 counts of lying to investigators.
November 2008 - Orange County States Attorney turns over nearly 800 pages of documents showing that someone used the Anthony's home computer to do internet searches for things like "neck breaking" and "household weapons". (Not commonly items of interest for retired people)
December 11, 2011 - The skeletal remains of Caylee Anthony are discovered in a wooded area approximately one-half mile from the Anthony home... the same general area where Roy Kronk had reported the "suspicious-looking bag" - forty months earlier!
Casey Anthony had proven herself to be a liar many times during the investigation. A "babysitter" that people had only heard of, but which nobody had seen! A half-dozen story changes to police, attorneys, her parents, her brother, etc. Her attorney, Cheney Mason, kept her off the witness stand for a very good reason! The testimony of a pathological liar doesn't carry much weight with a jury.
As was his responsibility, Mason tried to shift the blame to almost everybody in the courtroom, with the possible exception of Judge Blevin Perry.
This question remains - if little Caylee Anthony was not murdered by her mother, then who took her life? Another family member? A band of wandering minstrels? A mentally disturbed alligator? Or, is it simply that Caylee became the youngest suicide ever recorded?
Not being able to prove something, does not necessarily mean that it didn't happen -just that it can't be proven beyond a reasonable doubt. Or, in this case, a ridiculously unreasonable doubt. And, don't confuse "not guilty" as being synonymous with "innocent". Innocent is a statement of fact... not guilty is a statement of opinion.