The short answer is "Mutual Assured Destruction." If you jam the other side's radar effectively, the natural assumption is that you're attacking, so they'll attack in response. So you will set off a nuclear war with your first serious jamming attempt.
Under normal circumstances, you want the other side's early-warning radar to be working well so that they can see you are not attacking. There was at least one incident when a nuclear war nearly started accidentally owing to a radar malfunction.
The only time jamming makes sense is when you've already launched an all-out surprise attack, you start jamming just after you believe the enemy will be aware of the scale of your attack. At this point, it handicaps their ability to take defensive precautions. But that's the most use it can be.
