1. The S&P 100 companies were racist and previously preferred to hire white people over nonwhite people given equal qualifications. They changed their tune after the BLM protests, and are now race-blind in hiring, leading to 94% of new hires being nonwhite.
2. They were more or less already race-blind in hiring, but the pool of qualified labor skewed white. That 94% of new hires were nonwhite in a country that is roughly 3/5 white may indicate that equally-qualified whites were discriminated against.
3. They hired people in a race-blind manner, and ended up with 94% nonwhite new hires due to the composition of the labor market at the time.
4. Some combination of the above, or something else entirely.
> why their workplaces don’t reflect the country’s racial and ethnic makeup
This is a question with no answer, a game with no end. Virtually no subgroup in society accurately reflects the demographics of the whole, and it's fallacious to think that this happens solely or even largely due to discrimination.
I, for one, cannot wait to live in a society where the color of one's skin matters less and less over time.