As I understand it, it was largely due to the superior efficiency of the Russian engines over the bulk of US engines, at a vastly better price than the US optio

As I understand it, it was largely due to the superior efficiency of the Russian engines over the bulk of US engines, at a vastly better price than the US options of the time. Then institutional inertia takes over as switching rocket engines is apparently not just a trivial switch for a rocket design (but IANARS).