The prosecution didn't figure it out because they asked the Sheriff's Office for the "internet history" for June 16, and the Sheriff's Office just printed up the Internet Explorer history and sent it over. They should have also called the prosecutors and said, "Hey, by the way, we happen to know from computer forensics that Casey never used IE--she only used Firefox and her parents used IE--and we also know that the entire Firefox history was deleted more than once, and most recently it looks like it was deleted right after Yuri interviewed her and right before she was arrested on July 16, and we also know we only retrieved a small chunk of that history when you asked us to do that 'chloroform' search and we for sure haven't retrieved the history for the time period near the alleged murder, but it definitely should be sitting there in deleted space totally undisturbed since the computer was seized as evidence right after that, so do you want us to try to get the rest of it or nah?"
But they didn't call. So the prosecutors didn't realize they didn't have all the "internet history" for June 16. Meanwhile, the defense forensics guy DID retrieve the whole Firefox history from the hard drive, and the defense team ASSUMED the prosecutors had the whole thing, including the "fool-proof suffication" search. When the prosecutors said they might want to introduce the internet history for that day as an exhibit, they hadn't even seen it yet, but were just mentioning it as an administrative point for the court, no big deal. But Baez and his group thought THIS IS IT THE THING WE'VE BEEN WORRIED ABOUT and fought like crazy to keep it out. The judge was like, "give me a break you've had plenty of time and you must have known June 16 was an important date."
And then later, in the hall, Cheney Mason asked Linda Burdick (I hope I'm remembering all the names right) if they were going to introduce the internet history from that day, and she said no, after looking at it they weren't going to. And the defense came up with this crazy theory that the reason the prosecution left it out was that the search was done at 1:52 pm when George supposedly had seen Casey and Caylee leave the house, making him the only person at home on the Internet, so the search would have incriminated George rather than Casey. But in reality, the prosecution just had never seen the Firefox history for that time period at all.
And later, it turned out the search had been done at 2:52 pm (the software to interpret the time stamps wasn't updated to handle Daylight Savings Time until after the defense forensics guy had done his report), so George had actually left for work by then and Casey was at or very near the house based on cell phone pings. And the search was surrounded by other Casey-activity (MySpace, etc.), and as mentioned was on "her" password-protected profile and "her" browser.