Well I think that they've always been unfashionable rather than anything else. Partly because of the Pipe and Slipper stigma that was anything BMW in the days that the Bricks were launched, partly because they didn't conform to the norm; small (Peugeot-derived) car engine laid on its side, shaft-drive, seemingly minimal brakes (when the others were all floating discs the size of dustbin lids and multi-piston callipers) and tubular frames when others were going for extruded or welded aluminium beams.
They were always far better than their image and they are still very good bikes that can be ridden in any modern conditions. On top of everything else they are cheap to buy, to maintain and to run.