This is probably bone-headed, but here goes:
IIRC, when Hitler invaded the Soviet Union, his army had this huge natural barrier to contend with. They had to go north and/or south, since going through it was not practical.
The question I have is that most marshes/swamps are generally found relatively close to coastlines, where the land is flatter, but the Pripyat Marshes are pretty far inland. I know that a lot of times the ground around major rivers can be boggy or swampy, but the Pripyat Marshes cover many thousands of square miles. The closest thing to this that I can think of to this is the rivers that dead-end in inland deltas.
Is the topography in the Pripyat area flattish? I’d almost like to imagine it as a low spot–if it weren’t for the annoying fact that water has a hard time running uphill on its way out (thus the Caspian and Aral Seas.