Couldn't the federal government require it? For example, I know that a number of the most populous counties in Texas require vehicle emissions tests as part of the annual inspection. This being Texas, it's a good bet that they didn't voluntarily decide to do this. With the possible exception of Travis county (Austin).
There are a great many things the federal government could require, but the US is in a constant tug-of-war between federal powers and states rights. Similar to the EU vs its member nations.
Emissions standards for new vehicles is regulated by the EPA.
