The page you provided doesn't have a map, but looking at the one from wikipedia[1] makes me think it's just a heatmap of population and/or industrial activity. New jersey just looks exceptionally worse because it's a small state that is heavily urbanized.
Thanks for the map! Yes I somewhat agree, though superfund sites are designated as such because they’re particularly toxic and in need to remediation. NJ has a long history of industrial activity, though why it may be particularly toxic I do not know.
[1] https://upload.wikimedia.org/wikipedia/commons/9/91/Superfun...