The early mechanical regulator works like a normally closed relay. It energizes the field from IGN signal, when voltage reaches setpoint (about 13.8 V) relay opens. Without ground, no way to complete relay coil circuit, it won't open.
That would be the same issue with a transistorized version of the mechanical regulator. It still compares voltage to ground, and if there's no ground, voltage appears as zero, so maximum current is let through full time.
But with the '70 up regulators, the field circuit uses the same ground as the regulator. So completely losing regulator ground means no current through the rotor.
