It's rather arbitrary. The Egyptian word from which we get pharaoh doesn't actually appear as a title until the 18the dynasty. You'll find some folks who use "king" more often then "pharaoh" for rulers before that. When dealing with foreign titles vs. English titles, both have value. "Pharaoh" specifically designates the Egyptian monarch and all that's involved with that, while "king" describes their role to the best of out languages ability. If I say "King Tut," you can know he was a hereditary, absolute ruler with a divine right to rule without knowing anything about Ancient Egypt. "Pharaoh Tutankhamun" doesn't tell you that.