I think it's because we strive to be like him. Arthur is an example of someone bettering themselves. Especially in the more fantasical versions of the legend, such as when he draws the legendary sword and he shows that someone seemingly ordinary can accomplish the extraordinary. We as readers like the idea that we could all be the Arthur's of the world. Another reason they may be popular is how relatable the story is. In several versions of the legend, Arthur's beloved Guinevere cheats on him with his close friend. His success had put a weigh on his shoulders that showed him as someone who was not impervious to struggle, pride, and pain. By giving him these characteristics, it is shown that even a mighty king can struggle everyday as much as the rest of us.
