Why do pill bugs need moisture?

Sow bugs and pill bugs are crustaceans, just like shrimps, lobsters and crabs. They breathe with gills, so they need moisture in order to respire. Because of this you should have seen that most of the sow bugs and pill bugs spent more time in the damp rather than the dry soil environment.

Do Rolly Pollies prefer wet or dry?

There are two main reasons why pillbugs prefer moist and dark environments: Food and Surival. Food-wise, pillbugs eat algae, moss, fungus bark and all kinds of dead and decaying organic material. These can mostly be found in wet and most environments, thus it is only logical to find pillbugs near them.