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.
Is quite an assumption to make left wing movements and organizations in the West the defender of democracy. And another assumption to make the right movements the enemy of democracy. Also, take it from me who lived 15 years in communist Romania - the socialist states were very weak relative to the West.
Concentration of wealth and power was (and is) the highest in communist dictatorships - literally a handful (i.e. less than 5) people control pretty much everything in Cuba. North Korea is ruled with an iron fist by 1 guy - that is some concentration of power, right? In Communist Romania / East Germany power was concentrated in 2 people (a couple). In USSR power was concentrated in the 7 members of PolitBuro. In China power used to be concentrated in the hands of Mao Zhedong, now it seems it is concentrated in the hands of Xi Ping (but I could be wrong about Xi Ping. Maybe he shares some power with other people). I could go on forever, baby!!!
Capitalism has its problems but capitalism is quite fine all kinds of political systems - see German capitalism before, during, and following Hitler's rule.
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.