Age difference is part of it, but it mainly comes down to me not liking very imbalanced relationship. Just feels like one side holds too much power over the other, making the romance less genuine to me. Now, there are a couple different plot twists I could imagine happening that would change that and I have enjoyed some Noble x Servant stories before , but in this story with an Ore-sama type king and an innocent and slightly naive maid I just don't see it as the story for me.