I think one issue is that black BS graduates in the sciences and technologies are not particularly common, and they have been even less common in the past. This means that there are relatively few black scientists in industry, and a lot of pressure on industry to remedy the situation. Consequently, a graduating black BS scientist with the high levels of skills required to get into a good graduate program will be snapped up by industry.
I have only know a handful of black graduate students in chemistry, and they were without exception high performers who had very specific career goals in mind and had turned down lucrative job offers at the BS level to get advanced degrees and even more lucrative job offers at the PhD level. Several of the white students in the same programs either hadn't yet decided what they wanted to do with their lives or had been unable to find a job with just a BS in chemistry, although the rest were also very highly motivated students that didn't want the kinds of jobs they could get with a BS and wanted to run their own research programs, either in industry or academia.
The Asian and Indian population in grad school seemed to have a different mix of motivations. There were some that had come to the United States for college on a student visa, which would expire if they didn't continue being a student and they had gone to grad school either until they found a job or decided what they wanted to do with their life. There were some that had come to the United States for a world class education and would be returning to their home countries with a virtual certainly of a job as a professor or researcher in their field. And there were some that had come to the United States for the higher salaries here, but couldn't get a job or assistance with immigration without getting an American graduate degree first. (These were the immigrant students, those who had gone at least through high school in a foreign country - the ones that had come to the United States earlier seemed to be very much like the white population in motivation.)
Just some random thoughts from my own observations and friendships - your mileage may vary