Loyalty programs are marketing programs, period. So are casino rates. Nothing is earned or owed. I don't recall ever hearing a gambler claiming they cruise on Carnival because of the loyalty program. There is no benefit to the company to include casino rate cruises in the loyalty program - the casino can start their own program if they wish and could even cross brands.
Carnival has stated in the past that they aren't looking for cruisers who cruise for perks, but cruisers who like the brand. Translation - they don't want to get in a bidding war for fickle customers.
Since Carnival has already ruined their loyalty program by including essentially free cruises, and based on Carnival's target market has always been novice cruisers, it makes sense for Carnival to beef up the lower loyalty levels and let the higher levels saturate (leave them alone). They will attract and retain more of the target market. It makes no sense to do anything to the upper levels, including adding more, because Carnival's target market will never be able to reach those.