Theres been an article released in nature about this occurence actually.
Nature - Not Found
Dunno how the web links work in this forum, but thats it. The article suggests that the predator theory may be flawed, as there are many predators out there that will happily eat cicadas, and although it means there wont be one specific predator, they're still getting eaten a lot.
Another theory the article mentions is that it could be to prevent interbreeding between the species, reducing the chance of a hybrid occuring, and if it does occur using such large prime numbers reduces the chance of the hybrid breeding again. But im not sure how selection would work on this either.
Unseul
Give a man a fire and he's warm for a day. Set a man on fire and he's warm for the rest of his life....