Resposta curta: por causa de situações.
De Wikipedia : -
- Following the 1989 release of Indiana Jones and the Last Crusade, Lucas let the series end as he felt he could not think of a good plot device to drive the next installment, and chose instead to produce The Young Indiana Jones Chronicles for TV, which explored the character in his early years.
- When Lucas shot Ford's role in December 1992, he realized the scene opened up the possibility of a film with an older Indiana set in the 1950s. The film could reflect a science fiction 1950s B-movie, with aliens as the plot device. Meanwhile, Spielberg believed he was going to "mature" as a filmmaker after making the trilogy, and felt he would just produce any future installments.
- Ford disliked the new angle, telling Lucas, "No way am I being in a Steven Spielberg movie like that." Spielberg himself, who depicted aliens in Close Encounters of the Third Kind and E.T. the Extra-Terrestrial, resisted it.
- Following Stuart's next draft, Lucas hired Last Crusade writer Jeffrey Boam to write the next three versions, the last of which was completed in March 1996. Three months later, Independence Day was released, and Spielberg told Lucas he would not make another alien invasion film. Lucas decided to focus on the Star Wars prequels.
- In 2000, Spielberg's son asked when the next Indiana Jones film would be released, which made him interested in reviving the project. The same year, Ford, Lucas, Spielberg, Frank Marshall and Kathleen Kennedy met during the American Film Institute's tribute to Ford, and decided they wanted to enjoy the experience of making an Indiana Jones film again.
- Lucas convinced Spielberg to use aliens in the plot by saying they were not "extraterrestrials," but "interdimensional," with this concept taking inspiration in the superstring theory.
- Lucas found those artifacts as fascinating as the Ark of the Covenant, and had intended to feature them for a Young Indiana Jones episode before the show's cancellation.
...........few more draft and then it got finalized.
Parece que o filho de Spielberg teve uma mão em bombeá-lo para uma sequência que pode ir para o inferno do desenvolvimento.