Background: in my setting, there's a regional-scale nuclear war across most of Europe that primarily uses gun-type and boosted fission weapons. Essentially, mash this map and this map together, and assume a level of development/city-building roughly equivalent to 2020 IRL, other than the fact that nukes are rather new.
Then, hit every population center, military facility, shipyard, strategic command center, or other semi-important node on the fusion of those two maps with a fission bomb. Basically, civilization is dead in that section of Europe.
Now, let's say that most of the world mobilizes as part of a disaster-relief effort comparable in scale to WW2. Again, the worldwide level of development is somewhat similar to 2020, meaning that, given time, they probably will reconstruct that region. However, you can't have a post-apocalyptic story with Nazi sasquatch vampires (long story) in it without having a post-apocalypse, and so if that relief effort gets into place, it seems like things would be boring story-wise.
Therefore, I'd like to drag the post-apocalyptic period - i.e. from when the bombs drop to "bare minimum functionality" - out a little. I define "bare minimum functionality" as "there are structures set up for delivering aid, everyone generally knows where everyone else is, and things are generally not 'Mad Max' anymore" - think the beginning of post-hurricane cleanup and reconstruction, but across all of Europe.
Question: after a devastating nuclear war that occurs in Europe, why would it take years to decades for the area and its occupants to receive external aid?
I was considering using the Marshall Plan as a data point here, but, unlike IRL post-WW2 Europe, things aren't "shattered" or "impoverished" but still there; they're gone.