It may be that federal law offered some avenues for prosecution that state law did not. This occurred over the internet, using out of state MySpace servers, and that is likely how it was brought under federal jurisdiction, even though all parties involved lived in the same state. And federal conspiracy laws are a good deal more extensive than most state conspiracy laws.
Found this elsewhere: Lori Drew, 49, of O’Fallon, Mo., faces three counts of accessing protected computers without authorization to obtain information to inflict emotional distress and one count of conspiracy.