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.