From the article:
'Nintendo is, I believe, the only company that has basically refused to acknowledge the issue or demonstrate they are making any sort of effort on it,' said Sasha Lezhnev, senior policy analyst at the Enough Project. 'And this is despite a good two years of trying to get in contact with them.'"
If you read further into the article Nintendo's position is that the manufacture and assembly of its hardware is outsourced and they're not involved in choosing the sources for the materials their products are made from. Both Sony and Microsoft are ranked by the same group, and they have considerably higher scores.