To clarify, these are all stations owned by Sinclair Broadcast Group, which is the largest television station conglomerate in the United States. They've been pretty aggressive in buying up local stations, and have a reputation for exerting fairly strong editorial control and enforcing "must-run" opinion segments.

One study found that local stations bought by Sinclair reduce coverage of local events, increase coverage of national politics, and shift rightward in tone compared to other stations of the same area. http://joshuamccrain.com/localnews.pdf

To be honest, I'm not sure this shows lack of diversity of opinion - but it does show how consolidation of news media allows a handful of people to use local stations as sockpuppets for their beliefs in a way that hides the origin of the message.