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.