I'd say it is because of the higher-than-background visibility of various stereotypes, as they've been introduced to our respective (non-Asian) society through the media/etc.
Everyone
knows (FCVO 'know') that every Chinese business is a restaurant or a laundry or an illegal gambling and/or opium den with links to the Triads. Everyone
knows that when an oriental person gets threatened that they have uber-KungFu skills at their disposal, and because of the strangeness of the abacus in culture (compared with its virtual non-existence in ours) everyone's always seen super-proficient mathematical geniuses. And that's just the (largely) positive stereotypes. (To even things up, every Kenyan is a long distance runner, every Nepali man is a sherpa, every Argentinian is a goucho, every American from the Deep South is either a redneck or a river-cruising gambler, every Canadian is a mountie, and every Brit is a.... well, I'm not sure which of many possible stereotypes you'd put at the top of the list, but I'm also betting that they're all also 'true' (FCVO...))
Perhaps related, or at least in addition to this, in a discussion with an acquaintance yesterday about the
apparent predominance of doctors of Asian (subcontinental, rather than oriental) descent in the UK (which was his postulation)... well, some people notice them more, I suppose... but I suppose there's also the fact that immigrant families tend to push offspring a lot. Those that get pushed
and succeed at their(/their parents') chosen field get noticed. Those that rebel or fail despite trying submerge into the rest of the population and don't get counted
against the medical stereotype (but may get added to one of the others, with a perhaps less positive net worth to society). Doctoring is one of the traditional (perhaps now over-stale, but we're still seeing the fruits) aspirational path for the children of immigrants or, rather, of the parents, by proxy), etc. And to get into medicine, you need maths (along with the sciences), so that's one of the push-points.
By contrast "Tarquin", offspring of a hyper-aspiring middle-class couple of pretty much standard central-England stock, might have been encouraged to go into 'the city' (whether or not good at maths, for some reason that's not thought to be as necessary
),
or perhaps organic farming. And I imagine many Tarquins can still be found running second hand electrical retail outlets, living rough on the streets or any number of non-Banking occupations that also aren't small-scale agribusiness-related in any way.
What I'm saying (while
trying not to be deliberately provocative with regard to stereotypes... because that's what I'm saying they
are, so I hope you don't think I'm fooled by them) is that it's a predominant image that sticks. A meme that 'works' (FCVO...). It may help or hinder someone who falls under the racial or physical descriptor of the meme, although they also need to have the innate or drilled-in ability to make use of it. And if it all comes together, we hear of the young kid (of Asian/oriental background) factorising 10-digit numbers in his head (using a "mental abacus", probably), or of the person who thinks they've solved some other obscure Fields Medal-worthy problem, who grew up in poverty in some village somewhere before finding their potential in academia. This all makes a good tale and is memorable.
Stereotypes are stereotypes, then? Whod've thunk.
(BTW, I have
tried to be a bit more precise (where I want to be), given "Asian", to me and my British background is subcontinental India, rather than Japan/China/etc. Similarly, it's possible that my view of stereotypes (albeit at least
advised also by US movie representations of cultural tropes) is slightly askance from your (the OP and intervening posters') own group of known stereotypes, even once we get the geography sorted!)