Jonny Depp loves the Jack Sparrow character and said he would continue doing the movies if they were written. Orlando and Kira were not as attached to their characters and have no desire to continue with those roles.
Additionally, Pirates 4 builds off a small plot turn from the end of 3 - the search for the fountain of youth. 3 ends with Sparrow getting his hands on the map and 4 picks up on that journey.
The Elizabeth story at this time - goes to land and has baby. The Will story at this time - stuck on Flying Dutchman, cannot return to land for 10 years. If Will and Elizabeth show up in the 4th, it destroys the storyline created for them in the short scene following the credits.
Not to mention - I believe the idea is to focus on more specific Pirate experiences based on real life Pirate lore. If the 4th movie does well, Pirates 5 and 6 will be shot together.
Pirates 5 and 6
Thank you for link. I wonder what event they plan at DL. Hope maybe they will return to Will and Elizabeth in 5th or 6th movie if actors change their minds. There is still a lot to fix in their story, not the happiest ending ifyou ask me. Personally I do not like what they do with Story Line. It reminds me of TV Series with same main character and bunch of changes in every episode.
