There are two factors I'd like to share, both based on observations of many people from the former Soviet Union.
In the fSU, quite a few later-to-become-famous mathematicians chose maths because it was a creative field of study which was relatively ideology-free, - thus huge popularity of maths back then, and amazingly high numbers of famous mathematicians.
(an addition to the answer by Jose Capco) Russian specialized mathematical high school served as some sort of reservation for talented students who failed to "fit in" among their classmates from regular schools; entering those mathematical school, they escaped all bullying they would have faced otherwise, and were able to explore their creativity in maths without the peer pressure (which was rarely approving of "maths geeks").
