I think you'd have better luck qualifying where in the west you are referring to? This is a very open-ended question.

In Australia, my experience is knowledge of Pol-Pot/ the Khmer Rouge is equally as ubiquitous as Stalin but less known than Hitler in a general sense. This could be due to Australia's close proximity to Asia...

Rather than asking why Pol-Pot is less known, it is easier to understand why Hitler and Stalin are more known - World War Two. An event which directly involved the US/UK/Australia and probably the most widely studied contemporary event in modern history.