It also allows for the higher voltage in the primary as well. The higher (output) voltage is just an added benefit for a 12V system, hence the change to it.
A 6V coil (early) with the lesser voltage design primary, will not "put up" with the 12V 'spike' it gets every time the Ing. switch is turned on with the points closed, before the resistor heats/the starter engages.
If your resistor were a higher/constant value, then the voltage during the (starter draw) phase will be to low for a proper start (if any).
This is why there are 2 types (6 & 12V) of coils.
It maters NOT where (external/internal) the resistor is, which was the question.
