Ahoy, Craig!
Every once in awhile, Disney Cruise Line has two cruises that cannot be booked as back to back. I remember hearing about this issue in the past. Thank goodness I have a friend who is a travel agent, who was willing to lead me in the right direction to answer your question!
The reason that you are unable to book these cruises is a US Maritime Law called "The Jones Act". This states that a ship cannot transport passengers from one US port to another US port without visiting a foreign port that is outside of North America. Therefore, since the cruise from New York to San Juan only visits St. Maarten and Tortola and the cruise from San Juan to Miami visits those same islands, it is against the law for Disney Cruise Line to transport you on these cruises back to back.
If you are looking to try back to back cruises, you might consider sailing on the Disney
Fantasy out of Port Canaveral, Florida. The Fantasy alternates between the Eastern Caribbean itinerary and the Western Caribbean itinerary, so you would visit different ports each week onboard. It is one of my dreams to sail for two weeks on the Fantasy!
I hope this helps with your cruise planning! Please visit us again here at the Disney Parks Moms Panel with more questions that arise before you travel all the way from "Down Under"!
Happy Cruising, Tracy V.