I think it is partly because NYT and WP cover much larger ground and produce many more stories than Fox News. So a large number of visits to Fox News itself for stories belonging to small set of topics doesn't help Fox News with Google search results. Also, maybe reputation matters. Due to many reasons (and this is the elephant in the room I guess), Fox tends to be less objective than WP and NYT. Fox News also had to redact many fabricated stories in the past.