I think SS has also become 'trendy" because it is perceived as non-nonsense, back-to-basics, "bucking the trend" (though of course it is just another trend) of expensive, highly complex racing bicycles.
Someone somewhere said, "I don't understand how to shift and my shift-thing is broken anyway ... but hey, I can still ride." Someone else said, "Yeah, bike stores sell those expensive bikes with stuff no one needs just to make money." Someone else said, "Nobody needed shift-things back in Hemingway's time." Thus a trend is born.
Then you have threads like
http://www.bikeforums.net/general-cy...94-newbie.html where the OP states "I really am into the "hipster" city bike look. Simple frame without many logos, bullhorn or drop bars, a little saddle bag behind the seat." where function is absolutely in no way a consideration and the bicycle is almost as much a statement of personal style as a mode of transport, and whether or not gears are useful or might even make the person's life easier is not an issue---achieving a certain "look" is. (Nothing wrong with all that.)
Avoiding shifting has been dropped in favor of avoiding avoiding pretension which is in itself another sort of pretension ... but then, we all have some sense of style, and we all are "pretentious" in that we think our personal sense of style matters, when in fact every single person sees us through the lens of His or Her own sense of style anyway ....
We are complicated creatures and most of us don't know that much about what we are doing and less about why.
In the OP's case ... a lot of people do it because a lot of people do it. At some level we are herd animals.