It has always been my opinion that the reason for Hemingway's switch to stream of consciousness is because Harry is dying. It realy gets inside his head and sees his fear of dying, whereas before, when speaking to his wife, he puts up a brave front. It was rare for Hemingway to use this type of style, as usually his writing is short, dry sentences. It also serves to highlight the real world (where Harry is dying from gangrene) to his dreams of Kilamanjaro.