pea soup is a staple here in Sweden Best one I've ever had was made by our main cook when I did my military service, even my grandmother loved it (she tried it on a visitors day )
It's easy to make, plenty of ingredients available due to peas being crop that does well in the climate and soil. Peas are also good nutritionally and taste well enough that most soldiers won't complain about getting pea soup. So it's understandable that it would be quite common in Europe, especially in the north and east.