Individual states have the power to adopt hazardous waste management regulations that are more stringent than those of the federal government—and a number of them have done so.
In fact, in matters of hazardous waste disposal, the face of the EPA is likely that of your state—not the federal government.
Perhaps most significantly, individual states can differ from federal guidelines about what constitutes a hazardous waste per se, as well as how it should be handled.
Examples might be an industry-specific waste in a state where that industry is common; or a unique military waste in a state with a large military facility; or where the federal government allows certain waste to be placed in landfills, some states do not.
Also, different states might have different regulations about “lethality” or “severe toxicity” characteristics when determining if something is a hazardous waste; or they might add to the characteristics already in place per the EPA.