Lots of kidnappings get tried in state courts. Eve Carson’s killers were sentenced in NC courts. Polly Klaas’s killer by California’s.
Then, they probably weren't charged with kidnapping; they were charged with murder and tried under state law.
It's called the Lindbergh Law. Ever since the kidnapping of the Lindbergh baby, all kidnappings are presumed to have involved the crossing of a state line. This allows the FBI to become involved from the outset.