Space is a premium. They already don't have room for the ones they've got. Can't afford the space for another one that only two people can use.
I think it’s more likely that the designs are basically frozen. Surely USG could regulate this into existence for safety. They didn't seem to mind a much larger impact for spinning up the TSA.
It doesn't really solve the underlying issue. On long-duration flights, pilots have to rotate who is in the cockpit and who is in crew rest. They receive food and beverage service from flight attendants entering the cockpit. On very rare occasions, they are asked to enter the cabin and examine some part of the plane to determine if something is an emergency or not. Mantraps are perhaps a future requirement for cleansheet designs, but even that seems unlikely to me.