I've heard the story that a number of cities used streetcars or trolleys in the early 1900s, and the car manufacturers bought them out and shut them down out of fear of competition. Yet we still see them in San Francisco today. Why?
First, the conspiracy theory for decline of streetcars has very little truth to it. Here's my previous post on the subject.
San Francisco had municipal ownership of its streetcar lines from 1912, and so wasn't subject to the same pressures to stay profitable in the early 20th century. In addition, the city got essentially free electricity from the Hetch Hetchy project, so that tilted the scales in favor of electric traction.
But the real answer in San Francisco are the two tunnels: Sunset and Twin Peaks. Diesel buses couldn't operate through those, nor did the city want to use them for the J Church private right-of-way near Dolores Park. All the San Francisco streetcars other than the ones using the tunnels or private ROW were, in fact, eliminated in the 1950s and 1960s, just as they were in other cities. Running streetcars in mixed traffic with cars and trucks proved too frustrating, and—with the exception of Toronto—the only lines that survived until 1975 in North America (in seven cities) were the ones that ran through tunnels or used private right-of-way.