Most left wing movements and organisations in the West drew strength from the existence of strong socialist states, both materially and ideologically. These kinds of groups were a balancing force against the right wing/capitalist direction, which is inherently undemocratic, having as its logical endpoint the concentration of wealth and power in the hands of a few.

I think the true decline begun earlier though, around the Thatcher-Reagan era, with the erosion of all kinds of state ownership and control of our economy and broad attacks on organised labour.