The answer to the OP's original question is simple, race, but it has been fun reading posters dance around the subject.
Yes, one can argue XYZ about the site she is on, but that hasn't stopped other models who have only appeared on sites with pictures that cannot be posted here from becoming a sensation. One could also argue that yes, she doesn't always look she is comfortable in her shoots, but I could easily point to several other popular models on this site who don't always look excited in their shoots.
In the end all either of those two arguments is doing is dancing around the truth. It comes down to race and the fact that a black model of equal beauty as a white or Asian counterpart will not appeal to as broad a base of consumers. The content producers know it which is why you don't see many black models except for in niche genres that focus on ethnicity. Just look to this forum as an example and see how many women of color ever get mentioned, a disproportionate number considering how many are in the industry.
Notice I said race and not racism, big difference, although one could argue that hundreds of years of racism has impacted how race is used to quantify beauty even by those who would not typically be labeled as racist.