For Ashley, I guess it's because they couldn't think of any special move set for her. Saria also could be in the same category, along with the fact that Sakurai thought that playable Sage other than Zelda would be too far fetched. I wouldn't mind both characters.
(I'm still sore over the fact Ashley or Saria didn't make it into the game! >_< I mean Saria was long shot for sure, but c'mon Sucker eye! why u no like Ashley~ >_<