I may be mistaken and I went looking for where I might have read about it in my library but I believe Gould had a theory that the 17 yr. locust rythms has to do with the lifespan of the locust predators. That they evolved to take advantage of the shorter lifespan of the predators. Something like say some predator has a 15 year lifespan, and during this lifespan the locusts appear once, or possibly not at all. It may be a good food source but if it only appears once in the predators lifespan, there are better and more available foods out there.
I am sure that is quite a bit more complicated than that and I am probably missing some important points. I will keep looking to see if I can find more information.