(Contrary to popular belief, it was then a rather conservative city, not some sort of mythical artsy bohemian paradise. The "freaks," as they were then known, were a small minority of the population.)
This led to an anti-development backlash against the so called "Manhattanization" of San Francscio, which synergized with general anti-capitalist sentiment brought by the large number of leftist newcomers who arrived in the 1970s. The pro-development Republicans were routed entirely out of city politics by 1980, and rent control as well as a number of highly restrictive zoning laws were passed, aimed at preserving the picturesque postcard hillsides and views of the bay and halting all forms of physical and social change in the city.
The result is that most of the city has a "frozen in amber" feel to it. Making any substantial changes to the way anything physically looked circa 1950 is simply forbidden.
If you have to do repairs, you must demonstrate how it will not alter the appearance of the building. You must preserve the facade identically to the way it has been. You can often see gut renovations where the original facade is propped up on wooden stilts facing the street, and the entire rest of the house has been torn down to be rebuilt.
A current case in the local news revolves around the city granting landmark status to a 60 year old pine tree (not even a rare species) in some guy's backyard, so that he is legally unable to remove it: http://www.latimes.com/local/california/la-me-san-francisco-...