The having to lug your carry-on with you is my biggest turn-off. As a solo cruiser I can't park it with someone else in my party...I'm stuck with it. I didn't realize how nice it is to be allowed to drop it off on Celebrity until I did a totally sold out
RCCL cruise out of NJ in February and everyone was trying to find places to wait inside with their carry-on until the cabins are open.
My guess is the more family-centered lines figure there would be less compliance of "drop your bags, get your key card, and go" than on a more adult-centered line like Celebrity and that's why they don't offer it.