It was about health. You know, trying not to grow old and get sick and eventually die. Between the seventies and about 15 years ago, educated people in the US, the sort of people who got their toehold in the middle class by doing what their betters told them to do, would look at you like you were deliberately trying to kill yourself if you had eggs and bacon and white bread toast with real butter on it for breakfast. It probably peaked around the turn of the century.

The margarine section of the dairy case was bigger than the butter section. Imitation cheese slices were sold right alongside regular processed cheese - they're now completely dairy free and banished to the vegan food section.

There's a famous scene in "Sleeper" (1973) by Woody Allen where the character wakes up in the future to find out that everything that was considered bad for you is considered good for you again. Woody Allen was born in 1935 so had been around long enough to know these things go in cycles.