Because they were - by their nature - hidden, I suppose. We didn't see emaciated bodies being bulldozed into mass graves and never had to experience the day-to-day misery and paranoia of living in police states, so it's all a bit theoretical to most westerners. Add in the fact that its iconography and a watered-down version of its politics were rendered respectable by leftist parties in the west and you have a recipe for indifference.