The simple answer to this question is because there is a 1830s law on the books that states that foreign-flagged ships are not supposed to transport passengers from one U.S. port to another without stopping in a foreign country.

This requirement is primarily due to a 19th-century U.S. law called the Passenger Vessel Services Act (PVSA) of 1886. The PVSA prohibits foreign-flagged ships from transporting passengers directly between two U.S. ports. Since nearly all major cruise ships (Royal Caribbean, Carnival, etc.) are "foreign-flagged"—registered in countries like the Bahamas, Panama, or Bermuda to save on taxes and labor costs—they cannot legally sail from Miami to New York without stopping in a foreign country along the way. For a "closed-loop" cruise (starting and ending at the same U.S. port), the ship must stop at any foreign port (like Cozumel or Nassau). For a cruise that starts in one U.S. city and ends in another (like a Panama Canal transit from Los Angeles to Miami), the law is even stricter: the ship must stop at a "distant" foreign port (like Aruba or Cartagena, Colombia), which are ports outside of North America and the Caribbean. Violating this act results in a fine of nearly $800 per passenger, which is why cruise lines never skip these stops.