The reason blacks and hispanics are not employed in silicon valley is that there are not as many skilled in IT blacks and hispanics as there are from other race

The reason blacks and hispanics are not employed in silicon valley is that there are not as many skilled in IT blacks and hispanics as there are from other races in the United States. It's a reason, sure. If it were the reason, the percentage of black and Hispanic employees would be commensurate with the percentage of the "skilled IT" workforce made up by blacks and Hispanics. In addition, the stated decline of employed blacks and Hispanics would indicate a commensurate decline in availability of that skill set amongst blacks and Hispanics. Vast oversimplification probably isn't useful in this case. It's quite likely that there are confounding factors besides the reason you named.