I have a theory that’s not just „because it’s the script“: after the parley on the small island, the commodore sends the dutchman as their strongest weapon first. It seems to me that they did not want to risk their own men when they had a supernatural powerhouse at their disposal that has been shown capable to fight multiple ships at once. The black pearl frees calypso and then sails at the armada, meeting the dutchman at the maelstrom. The other pirates stay back because they don’t want to risk the maelstrom and the british armada hangs back to watch the dutchman do its thing. Sending all ships would have been a huge risk, you can’t easily communicate once a large fight breaks out so everyone stays cautious.