A lot has to do with percentages. Let’s say you cut back human produced levels from 12 to 8, a 50% reduction. However, if the average non-human level was say 100, you would actually be cutting the total level 4% which may or may not show a difference in health, however the cost of a 50% reduction in human produced levels may be a lot.

8 is not 50% of 12. But we're talking about massive numbers here. If that "100" is 1,000,000 tons of soot, reducing human-produced soot from 12 to 8 is taking 40,000 tons of soot out of the environment. And that's likely not uniformly distributed across the country: you're far more likely to have hot spots of human-produced soot, which adds up to tens of thousands of kids who don't end up getting (or are affected less by) asthma, thousands of adults whose lifespans are materially lengthened, and greatly reduced healthcare costs.

As noted in the article, the EPA estimated this rule change would have saved 12,000 lives per year.

For reference, 30,000 people are killed by drivers every year. 50,000 people die from suicide. So changing this rule is equivalent to reducing suicide rates by 25% or auto fatalities by over 30%.