The reason was to avoid tax advantaging sports cars or luxury sedans for executives. At the time SUVs were utilitarian and unfashionable so dividing by weight seemed an easy way to distinguish between "real" work vehicles and "fake" work vehicles.
Of course it backfired so now 6500 pound SUVs (and even luxury sedans!) are easily acquired.
Of course it backfired so now 6500 pound SUVs (and even luxury sedans!) are easily acquired.