♪Alone. She wants to be alone, to process things to a point of understanding.♪

@Star Cavalcade  
It’s probably hard to define a spell that knows if the act is stealing. If it prevented people from touching it it’d probably have to be everybody or specific people it prevents from using it. (like she could predict every person who’d try to steal it) Perhaps a spell could have been cast that allows only specific people to use it such as twilight and celestia. (but this could still pose problems in the future, plus it doesn’t allow for this convenient plot device)

@Star Cavalcade  
maybe she didn’t expect the Element of Magic to be stolen.

Why did Celestia cast a spell on the elements to make them immune to Discord yet still leave them open for anyone else to steal?