The same ports are visited most often because of several factors: contracts with the local ports and tour operators, offering the same itinerary because lots of new cruisers want to see those places, some port facilities cannot handle more than one or two ships. Caribbean cruises are marketed more for first timers. Remember that most people have never taken a cruise.
The Caribbean is largely boring to me these days. I cruise there for the warmth and the ship. I'm spending two weeks on the Liberty Of The Seas this coming December and I may not leave the ship at all.