Nazi sasquatch vampires (And their thralls) guarding the perimeter keep people out.
Admittedly, this is a large border, but this'll become a bit easier once they start resurrecting the nuclear dead, or intercepting foreign aid groups.
Foreign aid groups might actually make it easier to uphold the charade - "Oh, things are totally fine here, we're doing our best - they're even recovering!" or "Maybe send some more crew to the same port - we found plenty more needing help here.". Letting the outside world think they're helping while not actually successfully helping.
Worst case scenario - the bluff fails, and everyone knows their aid attempts fail spectacularly due to the vampires keeping them from succeeding, and if you succeed, well then the vampires can keep up the masquerade and effectively blindside everyone when they find out what actually is happening.
Other, non-nuclear conflicts break out in the rest of the world
The U.S. has an often discussed and criticized policy of being able to fund their army to be able to run a two theatre war. Even at the largest funding it had there, they're looking at 2 major conflicts and one limited conflict.
But the map you're presenting isn't that large a portion of the world; give the U.S. three major conflicts that aren't Europe. 164
You've still got all of Asia, all of Africa, and all of South America to choose from, let alone a possible Civil War breakout making them have an additional local conflict.
Even if Europe is a strong contender for foreign aid, you're indicating that civilization is basically dead in Europe - perhaps at a glance, the U.S. looks at that, goes "Well, that's horrible; we have to intervene elsewhere and make sure nowhere else ends up like that.". Basically containment of conflicts to keep them from having to aid another region is what's keeping them from aiding Europe at the moment. Especially if they're aware of the whole Nazi Sasquatch Vampire situation.
For bonus points - have the non-nuclear conflicts break out nearby nuclear countries. That can give you a potential read on the global news as Europe is hoping those conflicts don't go the way theirs did. 454