Sidecars remained popular throughout their invention to the end of WWII, at which point consumers wanted actual cars, not sidecars. However, once german automakers like Volkswagen started producing cheaper cars in the late 1950s, the sidecar's popularity declined dramatically.
Once full-sized cars were more affordable than a sidecar, businesses switched to the safer alternative; cars could be modified more extensively. As commercial use abandoned the sidecar and sales fell dramatically, rear seats made improvements in comfort, and a family sedan became the societal norm. However, as they lost popularity, their price became more expensive, and the sidecar was relegated to the world of recreational novelties.
In 2011, Harley-Davidson halted all civil production of the sidecar. However, it's interesting to note that the Secret Service still receives a special delivery from Harley-Davidson for the presidential motorcade.
Sidecars played an indispensable role in the history of motorcycles due to their contributions to warfare and commerce. Even as the memory of their wide use fades, they played an essential role in creating the society we live in, and for that, they deserve a round of applause. So tip your hat and notice those rare moments when you see one on the road.