All of the cruise lines are making changes to their Loyalty Programs. Royal Caribbean, who once had the worst, now have some good perks for their highest level members. Daily cocktail parties, private breakfast areas and continental breakfast in a private lounge, free laundry, discounts on cabins and other things. They have cut back on their onboard discount programs (coupon books) and there is less "free" stuff in it.
Carnival is now lagging behind in Loyalty rewards. Priority boarding doesn't isn't enough anymore. They need to increase the onboard amenities and discounts that other lines now offer.
I also believe that Carnival needs to increase their suite perks. Since they changed their categories and pricing they need to give people more incentive to book these higher end cabins.
I am not a Platinum member on Carnival but I am Diamond and Elite on Royal Caribbean and Celebrity and the Loyalty perks definitely influence my decision. There are other issues, such as food, that sway me away from Royal Caribbean and put Carnival above them on my selection list but the perks help to make up the difference.