Princess simply has decided to put fewer assets in the Caribbean. They've completely left the market during the peak summertime season and really don't put much into the Caribbean until November through April.
They've also moved ships to other markets since the Caribbean has become boring to many cruisers - especially those who have the time to do longer cruises since they tend to be older and more experienced. You don't find many young people or new cruisers on 14-night voyages, at least not as many as you do on shorter cruises.
Finally, most cruise lines have ceded the Caribbean market to Royal Caribbean and Carnival. It just isn't worth competing with them when they can fill their ships elsewhere.
14 night cruises are comparatively rare in the big picture. It is a diminishing returns thing. The longer the cruise the less money per day passengers spend aboard the ship - which is where the lines really make their money.