We don't. Stalin and Mao both made mistakes, the common assessment is ~70/30. Sankara and Castro made fewer mistakes. All led movements that drastically improved the lives of millions of workers and that is why they are held in high regard.
The problem is that most of us live under the dictatorship of the bourgeoisie, otherwise called liberal democracy. It's hard to have an objective assessment in the face of constant and pervasive anti-communist propaganda. Even you used the term "dictator", which is simply not supported by historical fact. On the other hand, there's not much point to propaganda for or against Napoleon.
"Made mistakes" is some very careful language on your part. To then follow that up with "the dictatorship of the bourgeoisie, otherwise called liberal democracy" is fairly deceptive.
Personally speaking, I don't think capitalism, socialism, or communism are inherently good. The only way any of them work is by mixing traits of others and regulation. Given that, they're all pretty arbitrary.
In theory tenured academic historians are supposed to be able to make neutral assessments regardless of which political theory is currently dominant. Whether that's true in practice is a matter of opinion.
You're welcome to examine pretty much any economic or health metric for China during and after the Great Leap Forward and the Cultural Revolution and draw your own conclusions.
My own unscientific view was that while Mao did an OK job unifying the country and (to some extent) fighting off the Japanese, pretty much everything he did after the founding of the PRC was varying levels of disastrous.
Subjecting your population to multiple genocidal events seems worse than 70/30 to me. Many leaders who we hold in middling esteem have managed not to murder tens of millions of their own people.
That's the dominant liberal propaganda, yes. It doesn't necessarily match the historical record, especially in different countries.
The same propaganda holds Churchill in high esteem despite the Bengal Famine and his general enthusiasm for imperialism and sympathy for fascism. There's a reason there are people with the first name "Stalin" in India, but Churchill is overwhelmingly hated.
We don't, we just use the term accurately: information spread with the purpose of convincing. It doesn't have to be incorrect, lots of propaganda is entirely accurate. We label our own efforts as propaganda, too.
The dominance of capitalism in most countries is not disputed. Is it far-fetched that capitalists would like to maintain their rule through ideology as well? The effects are readily apparent when comparing newspapers, school curriculum, public announcements, etc. from countries that disagree even slightly.
The problem is that most of us live under the dictatorship of the bourgeoisie, otherwise called liberal democracy. It's hard to have an objective assessment in the face of constant and pervasive anti-communist propaganda. Even you used the term "dictator", which is simply not supported by historical fact. On the other hand, there's not much point to propaganda for or against Napoleon.