Okay, first this : Position variables are used to position a coordinate system wrt another coordinate system.

Now, what are base scalars? They are variables that measure quantities in a certain frame. So, if we set the position of a coordinate system wrt another system using base scalars, that would be meaningless. Why? Well, what does is mean to have the child coordinate system position at say, (x**2, y**2, z**2) ? x, y and z are themselves the coordinate variables. How do you determine where the child frame is situated wrt to the parent frame? You can't. That's why the base scalars cannot be used to fix position of a coordinate system.