IMO there is a lot of things they simply did not think of when creating the entire ship and universe from scratch. When they revisited it later the ideas that came up since (Away Teams were the idea of David Gerrold, he had an entire book about what did not make sense in the TOS), the Ready Room was a logical addition. Having a place set aside for the captain to decompress but still be immediately available by the Bridge is a great idea, but I think they just did not think of it in the first show. All the later shows benefited from the expereinces and ideas of the prior ones, tuning the concepts and giving each show a special twist. The NX ready room was a must because everyone had seen them before, seen the scenes in them and expected to have it in the stories.