Two years ago we had a major explosion in the rolly polly population here - so many they would even crawl up the sides of coops at night. It was rather disconcerting! I worried about the chickens getting sick eating too many of them but it didn't happen. Like you say, maybe the isopods weren't tasty enough to be their snack of choice.

Of interest for folks wanting to cultivate a robust population, that was in the early summer following a very rainy spring and before a miserable-oven-bake-to-burn-drought of a summer. The winter and fall before we had relied heavily on dried leaves for in coop litter and all over the yards to combat mud and snow. Many, many, many yard waste bags were collected from curb sides and back alleys all over town all fall - we kept them along the north fence lines and coop walls to serve as wind breaks, insulation, and for easy access. Unfortunately the leaves also seemed to have brought mites which we battled that all spring. populations explosions of mites.