You might be surprised at the number of people who do find it interesting, and obviously, people aren't done talking about her. She is getting tons of offers. There is still a lot of public interest, otherwise they would shut it down and move on to another story.
How many people have sat glued to HLN since this trial started? How much air time has been devoted to this case for the past couple months? How many of the news anchors have been sent to Orlando to cover it nonstop? How much money has CNN paid out for hotel rooms, meals, salaries of all the anchors, reporters and camera crew to cover it? How many posts on these forums were devoted to what Casey was wearing every day in court, and how she wore her hair and how many times she got angry or cried? We can kid ourselves all we want to, but Casey has received more attention than many of the serial killers did.
There has been so much public interest in the case, and in Casey herself, I don't doubt that there would be enough people interested to make it profitable to publish her book. Don't kid yourself, if this book didn't have the potential to make money for them, they wouldn't be doing it.