For me, it's price, and satisfaction with the product! I love the shows, and the fact that Carnival has karaoke almost every night. Most of the other lines only have it once or twice during a seven day cruise.
The lovely Mrs. Jones (Vita) wants me to book a Princess cruise next year, (we're platinum there as well) so after looking into it, I find it'll cost me almost twice as much for a seven day on Princess as it does on Carnival! I like Princess a lot too, but don't know if it's worth double the price or not!