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: Cunard Queen Victoria January 2016
Next: Oceania Marina June 2016
Then: P&O Oriana August 2016
Then: P&O Oceana December 2016
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), Minerva, Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey