There's a lot of racism in porn. White stars are often signed to management agencies that tell these girls they can get contract studio (like Vivid) attention if they focus on the mainstream, white market. There are a lot of white fans who don't like chicks who do IR, so these girls try to play to that and advance their career.
That's why I love Sasha Grey and will salute her to my dying day. She became one of the biggest pornstars in the industry while heavily supporting IR porn. She was able to retire young and pursue other things, and now has a pretty decent mainstream career in terms of acting.
