The real answer to this is another question: How deep do you want to dig?

There'll be a dozen different answers depending on which cultural strand you're interested in.

Start with one: Rosemary's Baby was based on a novel by Ira Levin, who was also responsible for two other iconic 1970s horror films, The Stepford Wives and The Boys from Brazil.

What do all three stories have in common? Fear of reproduction, and... external interference with (or masculine control of) the sexual and reproductive process.

(According to a few interviews in the DVD extras on The Stepford Wives, the robot wives there were originally meant to be portrayed as highly sexualized, showing a lot of skin... but the creepier "Anita Bryant"-style floral dresses and sun hats came about once the director cast his wife as one of the robot spouses.)

Levin's novels coincide with a certain moment in reproductive science - the approval of Enovid, the first birth control pill, in 1960 - and its effects on feminism and Women's Liberation (by which I mean the specific political movement).

That's one thread.

There's also the way the theme intersects with the development of public religion (Billy Graham's crusades are giving way to Pat Robertson's 700 Club and the birth of televangelism)... or public education... or awareness of early child development... or fear of juvenile delinquency projecting to younger ages (compare with West Side Story and Village of the Damned... or the idea of a "generation gap" as the Baby Boomers hit puberty....