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

[Targan]

Perhaps the greatly superior accuracy of NATO's nuclear weapons resulted in a higher proportion of Soviet missiles being destroyed in their silos? Then again, you'd think that widespread NATO strikes on Soviet launch sites would escalate the exchange to a much greater level than described in canon.

There is a lot more specific canon info available about post-exchange USA than the USSR. Perhaps there are still many warheads intact in the remnants of the Soviet Union but their delivery systems are no longer functional? Also, the USSR covers a vast geographic area and we know that vast tracts of Soviet territory were no longer under central government control by 2000. Hundreds of Soviet nuclear warheads might be in the possession of dozens of factions of various legitimacy.

I don't know much about the durability of ICBMs or what sort of ongoing maintenance they require. How long after standard command and control and supply lines collapse before ICBMs become too dangerous to attempt to launch? Can they sit in their silos for a decade with little to no maintenance and still be safely launched? Obviously warheads designed for more conventional delivery systems would probably be more useful by 2000 (it's probably easier to verify the launch-worthiness of a cruise missile or a strike bomber than it is with an ICBM).

There are definitely canon descriptions of successful ASAT attacks, but not much to suggest that there were comprehensive space-based ASAT systems in place. I wouldn't consider it to be canon-breaking to suggest that limited, experimental space-based ASAT systems were in place at the start of the war. I had an NPC in my last campaign who was proud of the fact that he'd 'killed' a Soviet satellite with an aircraft-launched ASAT missile earlier in the war. Later when those kinds of systems had been expended/destroyed he'd been transferred to Europe as a USAF fighter pilot reinforcement before his F-15 had suffered a critical mechanical failure over Poland.

[schnickelfritz]

I believe the Soviets/Russians has some serious issues with the long term stability of their solid fuel rocket propellant. I tried looking to see why the rail mobile SS-24's were withdrawn and it would seem that something may have been amiss there. On top of that, the SS-N-20 in the Typhoon class had extensive issues during development with the motors and the follow on developments did as well.

It's possible some of the ICBM and SLBM fleet simply didn't work and were abandoned or destroyed as a result.

-Dave

[raketenjagdpanzer]

A post I made a while ago called "the care and feeding of nuclear weapons" touches on the maintenance issue. One component of modern nuclear weapons, at least American types, is polystyrene. Same stuff your $.99 picnic cooler or chick-fil-et cup is made out of. As it turns out it's a great way to quickly generate plasma to amplify a nuclear explosion, so they fill the hollow space between the actual core and the nosecone with it.

Well, the alpha radiation emitted from the core tends to melt the polystyrene, reducing it to a black crud that can gum things up, and so it needs to be periodically swapped out when it shows decay. If that's the case with US nukes there could be part of the problem right there.

[Olefin]

the US nukes are in the custody of MilGov - they didnt leave them in the silos or on delivery vehicles per several module sources - so it may be that the warheads are still in good shape but they lack the ability to deliver them

whereas with the Soviets they may be sitting in unattended silos or abandoned mobile units and thus neither the delivery vehicle or warhead works