Because they made spearing a serious penalty once it was decide spearing shouldn't be a thing in hockey.
And yet fighting isn't? Checking somebody into the boards is part of the game, but why should punching somebody not be considered assault?
This is a classic law school question. The answer is to argue that it is "part of the game", expected and normal. Same reason a boxer can't be charged or sued for assault for punching.
But why on earth should punching somebody be ok in hockey?