Cruise lines limit the number of comp players they allow on each sailing because it wouldn’t be a good experience for you if the casino was too crowded, all the tables were full, and you couldn’t find an open slot machine. So even if the ship itself isn’t sold out, there might not be any more spots left for comp players.
This is especially true for sailings offered on certificates or direct mail offers, since they tend to reach casino capacity faster. That’s why, when you see a great offer, it’s best to book it quickly!