Is there a good canonical explanation why there are so few humans of Asian descent in Star Fleet? Considering that over a third of the present-day world population lives in China and India, they're severely under-represented in Star Trek.
An in-canon reason could be the two bloody wars fought largely in Asia during the late-twentieth and early-twenty-first centuries in Star Trek canon. The Eugenics Wars were largely fought in Asia - Khan controlled an area stretching from Egypt to Australia, from memory - and the Third World War was fought between a combination of Western states and something called the "Eastern Coalition," which is implied in the novelisation of First Contact to have been destroyed even more than North America was. That same book posits Indonesia's emergence as a great power on the basis that every possible competitor no longer exists.
If we take the destruction of much of Asia as fact - and it is never explicitly stated on film or television that this occurred - then there is a very good reason there are few people of Asia descent in Star Fleet; most of them died hundreds of years before the events onscreen, with only people like Sulu and Bashir, whose families had emigrated to the West, being available to work for Star Fleet.