I assume it's mostly because of grace periods. See it yourself, try revenging someone you can revenge but choose to opt out of revenging them, then try to revenge them immediately again -- you will see you cannot because the game says "they are under attack" when clearly they shouldn't be. This is a grace period to prevent people from locking others out of the game with the "you are being scouted" message. So if you are constantly being encountered by the "they are under attack" message when trying to revenge then it is because they are probably in the raid pool and being scouted by other players or have been scouted recently. I'm not sure however if by trying to revenge them and encountering the "under attack" screen that you actually increase that grace period.
By the way, the grace period is about five minutes (from my personal test of it).