In an interview with the Daily Mail, Carole White, a UK-based agent who has worked with some of the world’s top models including Naomi Campbell and Christy Turlington, discussed the racism that permeates the fashion industry:

“Carole says the problem stems from the influential fashion capitals of Milan and Paris.

‘There, they absolutely don’t want black girls. A black model has to be a real star before you can take her there. They only take a black girl when the biz is buzzing about her.’”

The article came about after writer Liz Jones saw the resort collections for 2012 and noted that there were hardly any black models on the runways. She notes that the Chanel show was lily white, as was Yves Saint Laurent, with the sole exception of Marihenny Passible.
White explained that the decision not to cast black models can come from all angles:
The famous photographers are powerful and can make or break a model, often choosing who they want in an advertising campaign. ‘A lot of photographers don’t know how to light a black girl,’ [White] tells me.
It’s disheartening to think that the solution to this problem would be to simply eliminate an entire race of women from being represented in mainstream ads, instead of telling the photographer to go learn how to do their job, and in the meantime finding a competent person to shoot the pictures.
Because as it stands now, and as Jones points out in her article, as long as different races, sizes, shapes and ages aren’t depicted in fashion, women will continue to have to fight to feel good about ourselves, or to torture ourselves to conform to beauty standards that are totally unrealistic.