My first instinct was to say it started with McCarthyism when many of the genuine Marxist-Leninists were driven underground, and then over time replaced with COINTELPRO Trotskyists, while at the same time the "Secret Speech" made being a real ML now not just professional suicide due to persecution by the bourgeois state but also social suicide even among most of the "radical" leftist circles, particularly the more academic ones.
But really, the more i think on this the more i realize it goes back much further than that. Already in the 1930s you had a lot of bad takes in what was then the "Western left" on the Soviet Union. You could blame this on Trotsky but again this doesn't go far enough back, because even as the October revolution was happening there were a lot of social democrat type leftists who denounced Lenin and the Bolsheviks and sided with the Mensheviks instead.
So really for me it goes all the way back to the opportunistic betrayal that happened at the beginning of WWI when so many among the "left" jumped on board the pro-war bandwagon. The "Western left" never got over that split and the pro-war social democrats ended up essentially winning the struggle. Hence the German SPD, the British Labour party, etc. all eventually but inevitably ended up slowly degenerating until their final betrayal in the 1990s when they embraced neoliberalism.
So long as this traitorous social democracy defines the "Western left" all other leftist forces, even many of those who call themselves socialists or communists, end up being just the furthest left part of that same broader social democracy. They may be nominally opposed to the socdems and profess more radical views but they are fundamentally compatible and share many of the same liberal and western chauvinist assumptions about the world and modes of thinking.
As for the driving forces behind these dynamics and behind why opportunism has been so successful, yes i think you are essentially right, it has a lot to do with labor aristocracy and the elevated position that the Western working class enjoyed as a result of imperialism and colonialism. Their material and class interests were not completely but at least partially aligned with the preservation of the imperialist system and hence with the preservation of capitalism, albeit in a milder form with more handouts for them.