Decline of the Left?

[Agonized-Statism]

Tangential direction this thread took aside, I think the answer to the initial question is clearer than posters are making it out to be: 1992 was, according to George Bush in his state of the union address, "the year communism died". The stagnation and collapse of the Soviet Union and the integration of China into the world economy is treated as a discrediting of socialism, and triangulating third-way politics have become the "reasonable", "viable" alternative to neoliberalism since that time. Newly isolated socialist countries were forced to adapt as not to starve, all reforming away from the old-style socialism that the USSR supported to systems responding to global capitalist domination- in Vietnam, this manifested as liberalization, in the DPRK, this was hunkering down with juche and songun. The Latin American Pink Tide, which by all means would have been a revolutionary Red Tide if the Soviets were around or if the Chinese were still committed to communism, was largely an electoral democratic socialist phenomenon. The left was further crushed in the authoritarian environment of the 2000s' War on Terror. With the neoliberal "end of history" seemingly discredited by all the disasters of the late 2000s (instability in occupied Afghanistan and Iraq, the Great Recession, migrant crises, climate change, etc.), the left made a resurgence via the Occupy movement, European anti-austerity movements, and so on in the early 2010s. The high-water mark was 2016 with the Sanders campaign, IMO, but the same story played out globally: the left faced lingering resistance from declining social democratic parties that accepted third-way politics (including the Democrats, which while they were never on the left, had arguably been dominated by a New Dealer diet social democrat tendency since the Southern segregationists left the party and included leftists in their big tent from the 1930s until Bill Clinton). The split allowed the populist right to prevail.

Tl;dr, it's all because of the fragmentation of the left in response to the fall of the Soviet Union and the liberalization of China. For the left to resurge would take a rejection of neoliberalism that didn't actually pan out like people thought it would.