score:12
The Soviets encountered the Finnish snipers in the Winter War (the number one in your list is - Simo Häyhä - fought there) and saw their effectiveness first hand, so they heavily invested in training snipers right before and during the war. They also had (and still have) good starting material for training - the relatively large number of professional hunters (mostly in Siberia). Note also the Ворошиловский стрелок program, whose goal was general improvement in shooting abilities, not specifically snipers.
Other lists of snipers lists snipers from many countries, not just the Soviet ones.
Upvote:11
There were a number of reasons why the Soviet Union produced a large number of good snipers.
1) The Soviet Union had a population of 170 million, more than that of the United States, Germany, Japan, or Britain. They had a larger manpower pool to draw from than the other main combatants.
2) Many of the Soviet snipers came from Siberia, which was much like the American "Wild West," where people had to be wary about wild animals (and other people) all their lives, before the war began.
3) The Soviets had experienced the effect of Finnish snipers in the Winter War. Finland's population was 1/40th of the Soviet Union, so for them to have "one in ten" meant that they were "overrepresented.
4) For most of the war, the Germans were able to outfight the Soviets in combined arms operations. The Soviets had the advantage only in "urban" settings such as Stalingrad, where snipers could be used to best advantages. "Sniperism" was one of the few ways that the Soviets could reduce the long odds against them.