I am going to throw some random thoughts here. As someone who grew up in Asia, one of the driving factor is the limited supply/ high demand environment for almost everything including access to first class higher education. It is not enough to be in the top 10%. In fact, if you are not in the top 2%, you have to settle for tier 2. It is all about achievement, dedication to work, and lots of plain hard work memorizing copious amounts of data. Just as the sequoia grow tallest in competition with each other, so do we. That said, the truly gifted are just as left out there as they may be here, if not more so. Gifted underachievers have no place in the system.
Thought number 2, asians are the newest immigrants to this country so most Asian Americans are usually first generation. Their parents came here because in most cases, they were in the top 10% in their native countries and had the drive and desire to do more, work harder to improve their children's lives. So you start with highly intelligent, possibly gifted, driven families and the mix of genes and environment is most likely to produce more of that kind.