Mostly white probably means rural. These are undesirable hospital jobs. There are incentives for doctors to work in underserved areas, like rural hospitals, and those incentives are disproportionately used by J1s.

I'd like to point out that this commenter assumed these doctors did not earn their positions purely based on skin color. I don't see how this is functionally different than classic racism. "DEI" complaints often seem to fit this fact pattern. In particular, I've never heard a white man described as a DEI hire.