I live in the Bay Area, just a stone's throw away from San Francisco. The entire area is littered with hills. I find my multi-geared road bike quite helpful when the mechanical advantage is needed to traverse all of the hills I encounter here. It's absolutely nothing like my original home town back in Ohio, just southeast of the Great Lake Erie.
I used to often wonder why I would see so many single speeds in the Bay Area. Then one day, I had the opportunity to test ride a friend's old 80's converted Raleigh Record SS. I found that as long as I didn't try to take any really steep hills, I could manage quite well. In fact, it was really kinda fun to work up the small hills, just to coast down them again. However at my age, I wouldn't want to make it a Bay Area habit!
I then began to notice that most of the Single Speedsters all live either in the Mission district (which is flat), or near the Civic Center, in the downtown SF area (which is also flat). However, when you really think about it, much of the East Bay is flat when near downtown areas.
All I can say, is that the Bay Area, and San Francisco in particular, sure do have a heck of a lot of single speeds, to have so many darned hellacious hills!
If I lived in Chicago, Florida, or perhaps back in Ohio again, I would totally ride a single speed most of the time...