Originally Posted by Reign and Kisiro

When you first meet Zelda and there's the flashback of Zant taking over, it looks like everyone in the throne room is ready for him, prepared. However, there aren't very many guards. Perhaps Zant attacked the castle before that specific scene, and many of the knights and guards died in an attempt to stop him from reaching Zelda. The King was probably a part of this, as Kings have been known to join in during battles, and died. Zelda took over, but since there wasn't a propwer crowning ceremony, was still only called Princess.

It makes sense to me, but it's more Fanfiction tha anything.