Because it provides a clearer way of enforcing calling conventions. Calling a method with the wrong keyword argument raises an error automatically. Calling a method with hash arguments the method expect replies on the method manually checking for unexpected keys. In Ruby as in other languages it was error prone.

Positional arguments creates a dependency on the position of the argument. And this is a cause of bugs when calling a method with the arguments in the wrong order.

Keyword arguments remove the depedency on the position.