Because is largely exaggerated. All killing is immoral and I don't like utilitarian arguments myself but those dictatorships killed a fraction of the people than most think. Example, in Argentina where I was born, for decades it was believed that there where 30000 missing, but now the official number turns out to be a bit less 7000. Even people coming from the left-wing that, in an exemplary way did a self revision and lived their mature days trying to recover intellectual honesty have recognised that the 30K was a fabrication to amplify marxist propaganda https://vimeo.com/95210051