Parental pressure and hard work.
I don't think that cuts it as an explanation. That could account for why there are so frighteningly many BAD pianists of the typewriter kind (amateurs are out of scope here). This is usually a result of a forced approach to development in the wrong direction, mostly with the parents' "help".
Whenever I get yet another so-called prodigy, the first lesson is for the parents: One can only open doors for development, not push the student through a locked one (free after A. Schnabel). Healthy support - yes; peptalk - yes, but pressure - NO.
Let's have a look at some of the parameters that make a truly great pianist:
1) ease (no separation at all between technique and music)
2) personality (individual, beautiful sound)
3) musical intelligence (simplicity of all the lines holding the work together)
4) sensitivity and meaningful variety of detail
5) richness of texture (multiple levels of tone give a sense of depth)
6) controlled excitement
7) the ability to speak to an audience
These are all factors that CANNOT be explained by hard work and pressure. Hard work in the right direction, yes, but pressure is NEVER a key to great pianism. There are too many great pianists with a different story for that to be true.
Some things that ARE more important to explain the number of "more-than-average" pianists from certain countries or groups would be:
1) INNER drive to be the best or deep inner love for music, piano, etc.
2) the (musical and labor) culture of that people, the (musical and labor) culture of the family the pianist is raised in
3) quantity of population (in, for example, Holland with 16 million people, the chance of a great pianist being born is far less than in the mainland of China, with 1.3 billion)
4) high-quality teaching in all stages of the learning process
5) no mistakes in the learning process
6) system with severe competition, in which the less talented automatically fall out of the race,
etc., etc.
Paul