Media coverage. Period.
Parents kill their children every day. The media reports it for a few days, someone is arrested (or not) and the whole thing quietly fades away. There is simply nothing to talk about anymore.
In this case, the grandparents chased and courted the media and gave people a lot to talk about. Florida's Sunshine Laws allowing the release of thousands of pages of documents gave the media and the public a lot to talk about. Attracted by the media coverage, a whole host of bizarre characters showed up to make names for themselves which gave the media and people even more to talk about. Without all that information in everyone's face day after day this case would have faded into the woodwork months ago. The public would have lost interest, followed shortly by the bloodsuckers.
There is nothing particularly unique about a mother killing her child. There is nothing remotely unique about Casey Anthony either. She's just a common criminal. Heck, people are not even talking about her (or Caylee) most of the time anymore. They talk about Baez, bounty hunters, shady kid finder orgs, shady PR reps, bizarre grandparents, smuggled bracelets, suicide attempts and macabre dolls. It's a reality soap opera and that's why it's talked about.