Why are Black Mirror episodes in varying widescreen aspect ratios?

As an anthology series, there are very few connections from episode to episode. Most episodes have unique directors and the show has purposely used different visual styles from episode to episode to create the effect of different worlds. There is the common theme of the dangers of technology, but each episode exists in a different universe (with few exceptions like in Black Museum which puts some of the previous tech devices on display).

Using different aspect ratios adds to the jarring effect of episode to episode looking and feeling different from one another. In this Pushing Pixels Interview with Joel Collins, the production designer from all four seasons, Joel talks about making each show very different.

"Creatively, it’s like doing a series of pilots, with all the effort that goes into a long-running show."

and

"Every film has its own look and feel. So instead of the show having a Black Mirror look, we try to find the look that makes it right for that film."

The show gives a lot of freedom to directors to tell the story of the individual episode. Each director may have a preferred style that they bring to the show but more specifically, they likely choose aspect ratio as part of the story-telling device - do they want the viewer to feel claustrophobic or do they want you to feel a sense of wide-open awe. Regardless, each episode is its own film with a very unique look and feel.