Mayuka was a cute, playful character and Julie's child voice reflects those properties very well. Hikaru however, aside from a few scene in episode 1, isn't a cute, playful character at all. For most of the first season, Hikaru is a rather determined and serious young lady on a quest. Her desire to have the Magic Knights succeed on their mission is very strong. The second season goes even further, presenting Hikaru as an emotionally traumatized and grief-stricken young lady. I call her a young lady, because these are mature themes being presented, not children's games. And that essentially is the crux of the issue - Hikaru's character required a more mature presentation, not that of a child.
Unfortunately, that was one casting mistake that should have not been made, Hikaru is a central character, even more so in the second season. Not getting her right ruins the whole show, sad to say. Even though they did quite a few things right with the dubs, one big wrong was all it took.

Hikaru-chan's Magic Knight Rayearth Message Board

