I've always wondered if the answer to this was due to the culture and class demographics. The Current crop of tech leaders came from the era where middle class boys were bought computers by their parents as it was seen as a thing for boys to be into and not girls (I think this stereotype is still prevalent but far less powerful now). Computers just were not affordable to working class folks back then. Considering the poverty most minorities families faced then and still do now, it's sadly the case that they couldn't afford computers.
I expect (and hope) that in the next 10 years we'll see the distribution of tech leaders become naturally more diverse. Not because of any activist drive but simply because these kids are much more likely to have access to the tools they were denied in the past.