A lot of reasons! Living a hop, skip and a jump from the Elbe and being fascinated by cruise ships going by is one.
The second has to do with wanting to see the Caribbean. If I chose a land vacation to see that lush part of the earth....that would mean a lot of vacation time! Cruising reminds me of grazing at a buffet.
The third has to do with the Panama Canal. I´ve experienced the Kiel Canal and can only imagine how exciting the Panama Canal would be.
The fourth reason is as Thomas stated. Dh (hopefully) won´t be reached on a cruise ship. We´ve had two land vacations ruined by calls from the company. Provided dh can keep his mouth shut, I doubt that they will reach us!
Land vacations are also in our future. We´re hoping to get to either Namibia or South Africa next year as well.