Studies that looked at bias in the media found PBS to be the most centrist of the major news organizations. People on the extremes of the political spectrum see PBS as biased against them, as they would any centrist organization.
Another theory is that reliable corporate media is stocked with university graduates, and universities have also gone left.
> Would be interesting to see what this chart would have looked like 15-20 years ago.
Indeed, I think most of these were centrist news organizations 15 years ago!