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 Maritime Magellan Nov 2016
Next: P&O Oceana December 2016
Then: NCL Jade August 2017
Then: P&O Ventura December 2017
Golden, Grand(2), Caribbean, Royal, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Britannia, Oceana(3), Oriana(12), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(7), QE, Caronia(2),
Oceania Nautica, Minerva, Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey, CMV Magellan