First thing is that historians don't usually sit around ranking dictators on who was worse. So, the question is more about public reaction to the crimes of Hitler versus public reaction to Stalin.

So we can run through several possible reasons:

1. The world was at war with Hitler, but USSR was an ally that helped stop the Nazis.

2. The USSR was a nation that needed to be dealt with diplomatically post-war. Germany was already defeated. It's easier to demonize something in the past than the present.

3. The Holocaust happened in Germany, a nation that is considered as one at the front of Western culture. Russia before and during USSR was relatively poor and "backwards" in the eyes of many. It is more surprising and outrageous when crimes are committed by nations that are considered "more advanced".

4. USSR apologists would argue that Stalin's purges were necessary for keeping stability in the country. While the Holocaust was completely unnecessary and even detrimental to the war effort.

5. The race-based aspect of the Holocaust is really disgusting to a lot of people (and rightfully so).

I am not a historian but I have studied International Criminal Law (quite extensively) so I can give you a perspective on your question that I have found whilst studying the development of international law.

The first reason that the Holocaust could be said to have overshadowed other exterminations is the industrialised nature of the killing, and with it, the intent. Though at the time the Holocaust was treated as a crime against humanity ('genocide' itself was not a separate crime until the Genocide Convention 1948), it has, of course, been regarded as a genocide. Genocide requires "an intent to destroy, in whole or in part", which is a prime reason why the crime of genocide has a stigma attached to it above even that of crimes against humanity.

It is at most questionable however whether there was genocidal intent for Holodomor for example. The stigma of the crime of genocide would therefore raise the Holocaust above that of many other atrocities of the 20th Century.

A further reason that the Holocaust has had such an impact is that the atrocities of it were effectively showcased at the Nuremberg International Military Tribunal at the end of the war, a level of attention that has not been granted to many (or any) crimes since.

There is a certain politics to the construction of a 'kill count' associated with various people or regimes. The number you provide in your basic assumption - "20 million+ people" - does not reflect current research into the Stalin era history, and instead reflects an historical construction that dominated the West during the Cold War. We can speculate on what convenient purposes this sort of construct served in a Cold War context, but it is mere speculation unfit for the historical character of this subreddit. What follows is a lengthy answer.

To reply to your actual question, I have a few comments. First, it seems clear in the West at least that Stalin is not as overshadowed by Hitler as you seem to suggest. People in this thread seem to say that the USSR's atrocities are ignored or overshadowed, and I wonder how old they are or where they are from. In the United States, Stalin might as well be a synonym for Satan. His face is regularly placed beside Hitler's in various forms of political propaganda - even now, but certainly decades ago in the height of anti-communist hysteria. So in this respect, I cannot really understand the basic trajectory of your inquiry.

Secondly, however, and from a more rigorous historical perspective, there are vast differences between the Soviet regime and the Nazi regime. Putting aside a lengthy discussion of Nazi crimes - which, as I understand it, are recognized widely as mass killings and aggression that originated in central party policy - the set of Soviet crimes and crises of the 1930s are largely related to challenges of rapid economic development in a society rife with paranoia from top to bottom. In my view Stalin was as much a product of that broader political culture as he was its instigator. The degree of international pariahdom that seemed only to worsen through the 30s only provided more salient excuses for state excess. To get into more detail, read my next post.

In his book, Bloodlands: Europe Between Hitler and Stalin (2010), Timothy Snyder addresses your question in detail. He attempts to bring the extermination of European peoples into proper historical context and seeks to disabuse conventional, often English-language, interpretations that place sole emphasis on the Holocaust and genocidal policies implemented by the Third Reich during the Second World War. One of the reasons the USSR's egregious abuses and atrocities are not as well-known has to do with the idea that the Soviet Union was not only part of the Allied war effort, but also that the Soviets were an integral part in defeating Nazism and victory in Europe. In a curious way historians are only beginning to address, Stalin's deliberate policies to starve millions have yet to be appreciated fully in mainstream historiography.

By contrast, the Holocaust was industrial, wanton, as well as technocratic in its implementation. In addition, while the Holocaust was envisaged by the Nazi regime, its implementation was pan-European in nature. Without the substantial assistance from civilians in Croatia, the Netherlands, France, Belgium, the Ukraine, Poland, Belorussia etc. the Holocaust would not have transpired as it did. Media and popular histories tend to use Auschwitz as a symbol of the Holocaust's brutality, but it is important to note that the "Holocaust by bullets" killed millions in the east even before the crematoria at Auschwitz became operational.

So, let's get this out of the way first. They were both monsters, but comparing kill counts is problematic. More kills does not necessarily make you more evil, its not a completion. Another problem is that Nazi apologists tend to use that one as a defense.

Now on to why the Holocaust is looked at as worse. The Nazis industrialized genocide. Think about that. It hadn't been seen before or since. The extermination camps were built with the sole purpose to kill undesirables as quickly as possible.

This might sound callous but Stalin's kills can also be attributed to bad policy in addition to horrific purges. The Nazis were purposeful, efficient and systematic with their racial purification.

One way of thinking about this is that it is not just Soviet crimes that are overshadowed. Nazi crimes against Poles and Soviet citizens, including forced famine, politically motivated killings (all members of the CP were targeted for death) and far more indiscriminate terror against civilians, have always received less recognition than the Holocaust. The simple reason for this is that the targeting of the Jews was unique. It had a special logic and the way it was pursued had no precedent. Horrible as it may seem, people can understand the Nazis executing Communist Party officials and other political enemies, just as they can understand the Soviets executing Ukrainian nationalists/suspected collaborators. It has a certain logic. But going to great lengths to round up Jews, including women and children, from all over Europe, wherever they had influence...that is much harder to understand. It breaks the mold. The industrial nature of the killings is another unique aspect. But even if you take a more "traditional" massacre, wherein people are rounded up and shot, it is easy to see that the Nazis and Soviets were a bit different. The single worst massacre committed by the USSR was the Katyn massacre, but even that saw the targeting of a specific political group--Polish officers. In a Nazi massacre of Jews, for example at Babi Yar, it was all Jews. Women, children, the elderly. Deliberately targeting children based solely on their "race" is unique to the Holocaust.

Many scholars would argue that the context and intent of the Holocaust were what made it so historically significant, not to mention it's impact on our fundamental understanding of concepts like progress. Here's a post I wrote in another thread about this subject that I think provides a decent answer to this sort of question:

These kinds of discussions seem to arise not because other atrocities are so insignificant or necessarily unrepresented, but because the Holocaust has become the paradigm/benchmark from which we (in the West) view and judge other instances of genocide or mass killing. The reason for this, I believe, is that the Holocaust shook the West's fundamental understanding of progress, when before it was naturally assumed that evolving state bureaucracies and advanced technology could only mean good. In the Holocaust, you had a modern, industrial state, motivated by a peculiar ideology that our liberal paradigm still struggles to comprehend, which used the most advanced technology and sophisticated bureaucracies available to exterminate a specific demographic of people for no other reason than that they were Jewish. Also important, that state undertook the genocide at the height of its power, where most others like the Ottoman genocide of Armenians, took place when the future of the Ottoman state was in question. Less people may have died in the Holocaust, but it was the intent of the act, the time in which it was initiated (the height of German military supremacy), the machinery with which it was carried out and the enthusiasm with which it was executed that make the Holocaust so historically significant.

Now the Holocaust, as I've said, has become the measuring stick with which we judge/examine other genocides or democides. This model is pretty problematic because it imposes issues particular to the Holocaust (mostly the racial aspect) to other atrocities. For example, the Soviet Holodomor is frequently ascribed a racial dimension because it mostly targeted Ukrainians, but I agree with Mazower who argued that Stalin only targeted Ukrainians because they happened to live in the most fertile territory whose wheat he needed to fund his industrialization. The racial dimension of Soviet atrocities is also sometimes overemphasized in hindsight by minorities who were deported or starved during the regime, in an attempt to create a narrative regarding their unwilling relationship to Soviet Communism. Mazower also contends that Soviet national policies (forced deportations) were more of a continuity with Imperial Russian practices and are not comparable at all with Nazi racial policy because the Soviets still wanted those populations within their borders. At the end of the day, trying to compare death statistics is pointless because although Stalin's regime was responsible for more deaths, the vast majority of them were unintentional because of bad policies. On the other hand, the Nazi regime may have killed less people in the Holocaust (unless you want to count the ~30 million killed as a result of Nazi Germany's war), but the intent to physically eradicate undesirable demographics with no real economic or political motivations was present.

Trying to compare the two is also problematic because most comparisons imply a commonality between both regimes. This is called the 'totalitarian model', which has so many conceptual problems and ideological baggage that most comparisons will necessarily be simplified to the point of absurdity. This model's problems lie in its inability to explain differences in leadership, structure, socio-economics (which is the biggest difference!!!) because it suggests that Nazism and Communism are basically similar. I'm not suggesting that Nazi Germany or the Soviet Union not be considered totalitarian (as in, they both had a totalitarian will), but that trying to compare the two through it, especially through examining body counts, is pointless and doesn't contribute anything really insightful because it's essentially a moral criticism of non-liberal-democratic states.

One thing to add to the other excellent answers given in this thread: comparisons between Stalin and Hitler have often lead to heated debate between historians. A famous example is the so called Historikerstreit in the seventies in Germany. Historians on the right (the most important one being Nolte) started to argue that Stalins crimes were on the same level as the ones Hitler perpetrated. Historians on the left, most prominently Jurgen Habermas, disagreed, saying that this argument took away the uniqueness of the Holocaust (and, as others have stressed, the industrial nature of it). This debate was often going on in op-eds in German newspapers, so it was certainly known to the public. As often in the study of history, there is no direct answer to your question: historians simply disagree about it.

For more info you can also check out Norman Naimark's book Stalins Genocides (and especially chapter 7, where he "compares" Hitler and Stalin).

The Holodomor is only considered a genocide by 13 countries.