Because he was unshakeably principled as a communist and anti-imperialist, and during his leadership the USSR posed the biggest threat to the global system of capitalism that the world has ever seen. He could not be reclaimed for the purposes of anti-communist propaganda like Trotsky nor relegated to the status of a mere theorist like Marx or an idealist revolutionary like Lenin is sometimes (erroneously) portrayed. Stalin achieved too much in practice for the building of socialism, while the victory of the USSR in WW2 under his leadership gave socialism an immense prestige boost around the world.
In short, he scared the bejeezus out of the bourgeoisie for what he represented and what he could have inspired in people across the world had he not been smeared with the lies of Khrushchev and the anti-communist propaganda of the West (frequently borrowed directly from Nazi anti-Soviet propaganda), so they vowed to forever destroy his image and make sure no one like him would ever arise again.
Sadly, this ploy worked. Thanks to Khrushchev's speech of lies you even had other principled communists (at one point even Che Guevara believed some of the accusations leveled at Stalin) around the world start to doubt what they thought they knew about Stalin and the USSR which caused a worldwide crisis of confidence among communists and a massive split between those parties who accepted the Khrushchevite lies and those who didn't.
Meanwhile in capitalist societies anti-communist indoctrination raised entire generations to internalize the belief that Stalin was equivalent to Hitler and the USSR another Nazi Germany, which destroyed their communist parties as effective political forces and made sure that most remaining communists and socialists would have an almost instinctual aversion to the Marxist-Leninist line and practical revolutionary politics.
This led to Western communists retreating into the realm of purely academic Marxism as an economic and not a revolutionary theory, or into all sorts of schools of pseudo-Marxist radical liberalism (like the "Frankfurt School"), anarchism, ultra-left deviations, or just straight up defect to social democracy.
But i will end this on an optimistic note and remind everyone of what Stalin himself said:
"I know that after my death a pile of rubbish will be heaped on my grave, but the wind of History will sooner or later sweep it away without mercy."