There are two fairly good reasons for leaving Shelob until Return of the King.
1 - It gives Frodo and Sam something to do in the third book besides wander through Mordor.
2 - If you read up on the chronology, it isn't until Gandalf and Pippin arrive at Minas Tirith that Frodo and Sam venture into Cirith Ungol. Everything in the film is happening at the correct pace; it's just that pace isn't very easy to follow in the book.
Tancred
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\"HELP! I\'ve superglued myself to a flaming bowling ball!\"
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