Why did Hemingway, so well known for artful use of dialogue in his works, turn to internal monologue in The Old Man and the Sea?
The most important reason to use internal monologue in The Old Man and the Sea is that the main events of the story occur far out at sea, where there are no other people for Santiago to talk to. Without a dialogue partner, Hemingway would have had to show Santiago speaking to his skiff, to the fish, to the line or the sea, and while there are instances of dialogue -- Santiago's conversation with the warbler comes to mind -- it simply makes narrative sense for Santiago to spend more time thinking than speaking.
I worked the deep wells for a week and did nothing, he thought. Today I’ll work out where the schools of bonito and albacore are and maybe there will be a big one with them.
(Hemingway, The Old Man and the Sea, Google Books)
Another good reason for internal monologue comes after Santiago has hooked the Marlin; he needs all his energy to hold the line and keep the fish from escaping, and so the extra distraction of speaking, yelling, or other vocalizations would have weakened him. With little to focus on but the pain in his hands and back, and the pull of the line, Santiago thinks about his past as a young, strong sailor, and through his memories -- shared with the reader -- gains the strength to continue fighting.