But why? Where does that come from? Why is it different in Eastern Europe? My experience was that it was just another subject; those that were more diligent were as good at it as at other subjects (many were girls, as girls tended to be more diligent in general). Many were bad at it but they were bad at most subjects.

I would guess the 'being bad at maths is OK' or 'maths is uniquely hard' is a self-fulfilling prophecy, meaning many students don't try as hard as with other subjects.