Because it would have been found shameful and humiliating for all the British to have the one to pull the sword in the stone out (after countless men of all strengths tried) turn out to be a woman. So they (The people who wrote the history back then) changed the fact that their King was actually a woman. Today that wouldn't be odd considering we have women doing a lot of things, but back then women were not allowed to do anything a man would do. So all the British thought they would have been a laughing stock if the other countries figured out their ruler was a woman. So in short, it was because of sexism back then.