For anyone interested in the potential reasons: one of the most posited ones is that the creators of the tests are inherently biased. i.e. if someone creating an IQ test is a white male, chances are white males will have an advantage on the test.

This has some degree of experimental evidence behind it, I remember in one of my psych classes we learned about a study where test creators tried to create tests [and succeeded] where certain races or locales would score higher [primarily by having people in those places or people of that race create the tests]. It's very hard to create a "general intelligence" test that isn't biased in some fashion [and many say it's impossible]