GenZhou

because he fucked their nazi friends to ashes

Stalin got stuff done. Under his rule, the Soviet Union underwent a change from an illiterate agrarian poorhouse to a wealthy industrialised nuclear power, a transformation that, despite interrupted by the Nazi invasion, is legendary and to this day unparalleled in the world in terms of speed, scale, scope, and thoroughness of execution; along the way creating masterpieces in all fields of human endeavour, founding an immense scientific apparatus, developing the culture of the individual republics, establishing diplomatic relations with the whole world, winning the Great Patriotic War, and setting the global order for decades to come. No leader in human history can claim to be his equal.

This means he, and much more importantly, his philosophy and economics, is inextricably tied to the international left, and to keep him down is a necessity to keep us down. If anyone espousing his teachings were to lead another major country for an extended period of time, it would be a disaster for the bourgeoisie, not only in the West, but the world over. His combination of Marxist-Leninist philosophy, large-scale and long-term central planning, and a political emphasis on developing science and technology has proven lethal to global fascism back then, and it will prove lethal to global capitalism next time.

Because Stalin was the only Soviet leader after Lenin who was serious about socialism, aside from maybe Andropov but he didn't live in office long. Khrushchev and Gorbachev were both active revisionist while Brezhnev was just all around mediocre and unremarkable.

As a previous commenter said, Stalin was the last hardline ML to hold the position of premier. During and directly following WWII Stalin's opponents started distorting his memory, once Khrushchev took power following Stalin's death, he immediately started liberalizing the Union with revisionism. All of the premiers to follow weren't any better than Khrushchev and continued the process of "de-stalinization".

When the red scare started in the west, Stalin was still the premier and the USSR was both actively and passively engaged in ML movements across the globe, he was the obvious target for anti-communist fear campaigns, so a lot of the earlier red scare rhetoric focused on distorting him, his power, and the history surrounding his rise to power. Much of that propaganda was lifted directly from the Nazis, who spent nearly a decade making stuff up about the USSR.

So, it's a combination of following leaders being useless for anti-communist propaganda, and existing propaganda already being anti-Stalin.

Because he was by far the most successful (though Mao comes a close second, and so does his demonization in the west) and the biggest threat to the global capitalist hegemony. No other person perhaps in the history of the modern era has ever scared the ruling class as much as Stalin, along with the Soviet Union's and the international socialist movement's unparalleled success under his leadership did. Stalin instilled a sort of generational trauma in the bourgeoisie, it was the closest they ever came to completely losing their grip on the world.