Despite the fact that he defeated him, why is stalin less popular than hitler?

a history teacher of mine once recounted an event that once happened in his class while he was giving a lecture on ww2. He noted that the history book had excessive information on hitler, mussolini, hirohito, roosevelt and churchilll, but had no mention of stalin who is often accredited by historians for playing the primary role in defeating the nazis. Baffled he decided to ask his students the following question:"what is the name of the soviet leader who played the most important role in defeating hitler?". The next thing that happened ,according to him, was that the class fell into pin drop silence. No one knew who was stalin. Apart from my teacher's experience , i have also seen in real life that people clearly remember hitler but when the name of stalin is mentioned, they seem confused and some outrightly ask me who he is. In you opinion, Despite the fact that he defeated him, why is stalin less popular than hitler?