The state is trying to comply with US EPA regulations.

If the US EPA (the feds) designate a county as a "non attainment" or "severe non-attainment" area, that county is subject to emissions inspection. It's not directly tied to population-the feds don't mandate that all counties with X number or more population get emissions inspection, but higher populations generally go hand in hand with more cars and more pollution, so it works out that way.