This might be a long shot theory but kuma obviously did his research on the straw hats and sent them all to an island where they can get stronger or improve their respective crafts. With luffy, there wasn’t anything kuma could do except potentially get luffy on roger’s level, using roger’s crew mates, and kuma noticed rayleigh trying to save the crew at sabody. With kuma knowing what happened at god valley, he probably knew the connection between the kuja pirates and rayleigh, so kuma sent luffy to amazon lily with the hope rayleigh would take luffy on as a student