That's just the way the Game Designers wanted it I suppose.
Besides, Magnezone has sturdy and magnet pull already, it would have been nice to cut out it's 4x weakness to ground though. Or at least have it as a DW ability, I guess not.
Perhaps the game designers thought that Gengar could have the ability to levitate on command and didn't really have a good substitute at the time. Unless you ask them directly no one really knows.