One neglected characteristic ties all these images together: They are all horizontal.
It sounds trivial, but going wide helped differentiate TV key art as its own medium, distinct from book covers and movie posters. And because these images appear on streaming platforms, they are unencumbered by other marketing copy, like taglines, cast and credits, and multifarious blurbs.
There is a simple purity to key art.
I remember scouring the web for key art when I used Plex over ten years ago. It is such a specific category of design — bracingly simple and evocative — and this is a great post and collection from Sorgatz. I hope this is not yet another thing to add to the list of creative pursuits A/B tested to death.