Why do you think ir romance and erotic lit has yet to go mainstream? why do only authors of color seem interested in telling stories with non-monochromatic casts?
Erotic lit is in bookstores proper nowadays. Books-A-Million especially has dedicated bookshelf space for it. In terms of IR Romance, the market is still considered too small for it to go mainstream (as I'm assuming you're not talking about the very popular white woman/native/arab man. Those get bookshelf space in bookstores). And I think authors of color are more used to telling stories with diverse casts because they have to deal with diversity every day. White authors don't necessarily have to lead diverse lives so those aren't the stories they usually tell or maybe are even aware they can tell. But that's just based on my limited observations.