They started this strategy when departing from Australia. It's a chance to experience the ship itself for new departure ports. This is doubly so for the Adventure because it's going to be so huge that you'll need that time just to try to experience most of it. I imagine they will add other ports after two years or so.
Mama Disney does know what's best for me. Why offer a choice to visit a port when you could just stand in line for a character meet instead?