When I worked in television in the early 1980s there were rules limiting how many television stations could be owned by a single company, both nationwide and in a single market. I thought this was a good idea at the time and I still do. Unfortunately the rules have changed, and while you might think you're getting a diversity of channels over the air and especially on your cable system, in reality those channels have far fewer owners than you might expect. (The same is true of radio.)
Currently the largest single owner of local television stations in the United States is the Sinclair Broadcast Group, which is in the process of trying to buy, for $3.9 billion, Tribune Media, greatly expanding the number of television stations it owns, making it even more dominant, and reducing competition. Opposition comes not just from groups like Common Cause but from many parts of the business community. The Federal Communications Commission would have to approve the merger, but the current FCC, which has already reversed its longstanding support of net neutrality, may well be inclined to approve it.
One reason is that Sinclair's management is extremely political, far to the right of Fox News, and routinely requires the stations it owns to broadcast hyper-partisan segments on local news broadcasts.
You can see examples in the clip below from Last Week Tonight with John Oliver originally broadcast in early June:
See also the August 7 article Variety by Cynthis Littleton.