Definitely culture. Asian students are generally highly motivated, much more so than their western counterparts.
I've taught in Japan (public and private schools), South Korea (a public school), the US (private and public schools) and Australia (a private school).
In my personal experience as an educator, Asian students when taught in Asia are not any more highly motivated than students in the West. In fact, I'd say the Japanese and Korean systems work by promoting
less effort overall, but making sure that effort is more narrowly focused (thinking of mathematics, history and social studies here in particularly) and directly relevant to what will be on the final centralised exams.
Also in my personal experience
some Asian students, when in Western nations, are pushed harder by their parents to be academically successful. This is
not the same as Asian students being more highly motivated in general. I'd say there is a greater proportional pool of highly performing Asian students, but that number of poorly performing students in Western schools is roughly representative. It's in the middle of curve where Asian students are absent.
Speaking generally about the Japanese and South Korean education systems. From kindergarden all the way through to the end of high school, these place an emphasis on teacher-led rote learning, in-class repetition, high volume of work in certain subjects and collective responsibility (teachers and student body) for poor results.
The US and Australian education systems are quite different. They initially emphasise rote learning, but increasingly move towards more critical thinking skills, particularly in the final two years of schooling. They also embrace wider learning, including non-academic skill sets (clubs, sports, hobbies, arts and cultural groups) that are typically viewed as a 'waste of time' beyond about middle school in Asia.
Australian schools (I don't have too much US schools experience beyond middle school)place an emphasis on individual academic accomplishment and merit and while volume of work is very high, it's generally spread over more subject areas and is wider within the subject area itself.
Since I taught in Japan in 2004/2005, I know that the curriculum has been broadened somewhat, particularly in modern history courses - which has been somewhat controversial with Japanese right wingers and traditionalists.