So why not continue to use the RD-180 or, if needed, license the
RD-170?

Reducing our dependency on Russia would be a good enough reason by
itself, IMO.


I've lost track of this stuff somewhat, but doesn't P&W have license
(and presumably the blueprints and manufacturing techniques) to
manufacture RD-180s independently of Russia? Pat's point about the
engine being designed for somewhat more demanding requirements than
needed now should be noted.