There's a lot of repetition about "reasons" in the thread, however, I found an interesting historical fact that seems to be the start of this, as with everything someone has to set things on fire before the crowd follows. Probably there's more of the same to be found if one digs deeper, but this seems to be a good start.
Seems to me Robert Frank was an important trend-setter to this photography fashion, probably this opened up for others to start working with this kind of expression as well.
"His style of photography was also under criticism. Popular Photography said the images were “meaningless blur, grain, muddy exposures, drunken horizons and general sloppiness.” It was a completely different style than what people had seen before. Before Frank came along, the composition of photographs had been straight – either horizontal or vertical and the subject was always obvious. You knew what the photograph was about. Frank changed that. He was not looking for technical perfection. His pictures were messy and grainy, and he purposefully used obscure lighting."