All-white racial monoliths in the relevant institutions were the problem. Diversification is the removal of that problem.
The mere absence of a discriminatory hiring rule does not fix a problem that wasn’t caused by rules in the first place. Since at least as far back as FDRs 8802 executive order, equal employment rules were already in place within the federal government in some form or another. Yet the problem had to be revisited again in the 60s with affrimative action.