Why are woodlice worse than earwigs.
I would say - humidity difference at hive top - it will tend to be more damp for woodlice, but that is purely a guess as I get earwigs but not woodlice.
If the cover board is secure (fitting well and no gaping holes), there is no real problem with a few sheltering (or even living) in the roof - as they cannot access the hive from there, anyway.
Flemage, you have slugs and snails in your roof? You need better fitting roofs, I would suggest. Certainly wasps should not even get a chance to enter a hive via the roof!
RAB