I mentioned this in another thread some time ago, but if you look at Carnival's fleet and RCCL's fleet - which looks more modern? In 2006, Carnival debuted the Liberty - a very nice ship, but it looks ancient compared to RCCL's 2006 entry the Oasis of the Seas. Nothing CCL can do will make the Liberty have anywhere near the amenities of the Oasis. Carnival's debt load means they do not have the resources to turn over their entire fleet, so they're doing the next best thing - taking over ships from other lines they own.
The only way the Liberty can compete with the Oasis is on price. But the Venezia or Celebration have plenty of amenities to keep most cruisers happy, so Carnival can compete less on price and more on amenities with these better ships. That allows them to pay down debt.