I'm guessing that on Health & Safety grounds each ship has a maximum allowed occupancy, and therefore Carnival didn't go to the extra expense of fitting these rooms out with the extra berths.
It's ashame as that would have allowed more options to families, but from Carnival's point, it certainly would have increased the build cost & time.
I'd be interested to know if my 'guess/feeling' is right or not.
Last Cruise: Royal Princess August 2015
Next: P&O Oriana October 2015
Then: Queen Victoria January 2016
Golden, Grand(2), Caribbean, Royal, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Britannia, Oceana(3), Oriana(11), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(6), QE, Caronia(2), Minerva, Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey