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.
I’m not sure how far inland you think they need to be. The Okefenokee of Georgia (and a bit of Florida) covers over 600 square miles and is around fifty miles from the Atlantic coast. There are more than 2.5 million acres of wetlands in the Dakotas. Much of that is of the “pothole” variety with just a patch of swamp in a river bottom, but a substantial portion of it is a large area of interconnected lakes and streams covering a very large contiguous region.
There are also several similar regions in Canada, although several of them might get shuffled in to descriptions of taiga, generally, and may not meet everyone’s definition of marshland.
As to the land surrounding the Pripyat, the lands south and of the Baltic in northen Poland, Belrus, Estonia, Latvia, Lithuania, and Western Russia do tend to be quite flat (not claiming there are no hills, but that the general layout is more likely to be flat than hilly.