It is considerably better for the enemy to trade the Capital territories of one of your Allies than it is for you to take it back only to loose it your next turn.
Example. On turn UK6 England takes back France with a couple infantry. All the French territories in Africa you have liberated return to French control and yay France gets some free infantry. Italy purposely does not take it back on their turn. France then collects their 19 bucks on France6.
Well come Germany 7 they are going to just roll back in and retake it and collect the 19 bucks. Also the French territories England and America may have liberated from the Italians or Germans do not collect you money until France goes again.
You must retake them capitals with sufficient force to hold them through the next attack or have a darned good reason its soo important.