I believe the appeal of Midwife is its total focus on the unadorned reality of life during that era in post-War, economically depressed England and the emphasis on simple human values the plots and performances project.
A similar appeal was projected in the 1950s situation comedy, The Honeymooners, with Jackie Gleason, Audrey Meadows, and Ed Norton. The background in that situation was a rather drab tenement flat occupied by a city bus driver and his wife and devoid of the materialistic accouterment typically seen in contemporary tv situations. There was no designer kitchen in the Kramden's flat, no lavish living room, no "American dream" ranch with garage, lawn, and picket fence. No elevator apartment building with doorman and fashionably dressed neighbors. There was nothing in the background of Ralph and Alice Kramden's very ordinary 1950s working class lifestyle that separated them from the fundamental reality much (most?) of The Honeymooners audience could relate to.
The Midwife is about simple human compassion as it occurs in a simple, unadorned, human reality.