What is the main reason that attitudes are more often revealed in spoken rather than written language

The main reason that attitudes are more often revealed spoken is because of tone. If you're writing, your tone isn't portrayed as well as if you spoke it. 

The answer is "he was bad enough in all conscience, but the devil himself could not temp him to turn slave-trader. satirical is a synonym for sarcastic. so when the author says that tom walker could not be tempted by the devil he does not literally mean "the devil" but he means that he is too morally upright to become a slave-trader.