Here's an idea that doesn't break canon too badly

[Olefin]

The question may be more why do the Soviets seem to have so few and the US still have a bunch?

If you read some of the US modules it has mentions of MilGov removing nuclear warheads before CivGov could get to them from various locations - but with the Soviet side it sounds like they are almost all gone (for instance see how the Soviets react to three missiles and warheads still intact on the Soviet Sub in Boomer)

Considering other mentions of Soviet warheads missing their targets or malfunctioning did they have to use almost all of their arsenal due to so many misses and malfunctions just to get the results they got (minus whatever the US or Brits or Chinese took out first)?

Or did possibly the French hit them back and take out a significant amoutn of their remaining arsenal after the Soviets nuked the French refineries and ports (and thus explain the relative lack of remaining Soviet warheads)?