What is the cause of disparities among people of color or mixed race when trying to find donor stem cells in the donation paradigm?

Donor pools are currently mainly solicited in really highly-concentrated white areas and urban areas and generally high income areas. As a result, the donor pools are skewed white. So that's why it's really hard to find a donor. 

On top of that, if you're mixed, they generally try to align the donation ethnically to some degree. I knew someone who was a quarter Black, a quarter white, a quarter Mexican, and then a quarter Asian. It was four different types of ethnicities that were in the mix, and they just could not find a single match that would be able to match well enough. Because it's HLA matching, and generally tissue patterns and things like that are inherited, so they're usually aligned ethnically as well. A good match generally needs to have all the same ethnicities represented.