I can't speak to why the government decided to do a defence policy review and not a foreign policy review. It's a decision above my pay grade.
The United Kingdom did their policy review not just in the context of a foreign policy review but also in the context of an entire government spending review. They did an A-base review of every department in the government; defence was only one part of it. That may have had something to do with part of the logic behind why they chose to go the way they did.