Can't believe they skipped the Cold War, and jumped straight to the Vietnam War.
The Cold War was not an actual war, rather a disagreement between the Soviet Union and United States. Whereas brinkmanship was common during the Cold War (getting close to war without actually going to war), there were no actual events that would be fun to partake in for a first-person shooter. The Berlin Wall would be the only thing in which could be used, but that event is not nearly significant enough to be given a game.
A game concerning the Cold War would be great, but certainly not for a shooter-based title.