Logistics? Language barrier? Convenience? Ethnocentrism? That may be so, but in tennis it isn't uncommon. So why is the situation in badminton drastically different? Discuss.
You probably mean from different countries, not races, but one simple reason is you can't enter the Olympics as a pair with different nationalities.
The Olver/Agathangelou pair are different enthicities from the same country, and Hendra Setiawan used to play with a Russian girl whose name escapes me (could be Russkikh). Gill Clark mentioned during AE commentary last week that she once played with a Korean in XD when her usual partner had to drop out.
As I see it there are already a few mixed ethnic pairs about. I can think of Canada's Michelle Li/Alex Bruce and Derrick Ng/Alex Bruce. How about the USA's Eva Lee/Obanana? Obanana was originally from the Philippines. As for Tony Gunawan and Howard Bach, Tony is a Chinese initially of Indonesian nationality who's now a US citizen, while Howard was born of Vietnamese parentage and later became a US citizen too. Malaysia had up until last year a MD pairing of Zakry Latif (a Malay) and Hoon Thien How (a Malaysian Chinese). Singapore has imported and given PRs/citizenships to foreign players who now play for the national team eg. XD pairing of Lei Yao (born in China) and Chayut (sounds Thai to me), and the WD pairing of Lei Yao and Shinta (former Indonesian player).
i think the major factor is racial demographics. in countries like China, it is already hard enough to find a non-Chinese, not to say one that is badminton player and good enough to play internationally. similarly is the case for European countries, like Denmark, UK, and most Asian countries like Japan, Korea, Taiwan. the majority of the population of one racial background. in other countries with a more mixed racial demographic, like US, Canada, Singapore, Malaysia, the chances of that happening is more likely.
Yeah... my wording wasn't too great, but I think you guys get the idea that I meant nationality/ethnicity.
I would hazard a guess that you don't see mixed nationality teams at an international level since most teams are playing for a particular nation and thus, usually hail from there.