Why can't the predators evolve to have the same cycle as the cicadas, with a prime number of years?
It's important to note that, for the cicada, they are not a staple diet but rather an opportunistic one that occurs only once in a few generations. This means that the predator's cycle would have to skip generations in order to be effective. This is unlikely, given that that cycle would have no evolutionary advantage in the intermediary, and would thus be diluted by more pressing concerns.