I think the time frames are more important to look at. If we compile a list of interventions led by the USA, the majority of them have been democracy tampering and more pointedly, the majority of them are post-WWII. Stalin and Hitler both led their purges pre-WWII (obviously).

The USA dabbled in interventionalism pre-WWII with the Monroe Doctrine, but that was essentially just the American military bullying small Latin American countries into subservience. At the time, no country intervened in another due to humanitarian crises. In the wreckage of WWII the international community decided to essentially tolerate humanitarian intervention as a legitimate casus belli. This is where we get things like the Korean War, the Vietnam War, the Soviet-Afghan War... the underpinning of all of these conflicts is neoimperialism and inferior opponents.

Ask yourself why the USA doesn't presently intervene in North Korea where we have strong documentation of the Kims committing atrocities. The answer is simple politics - North Korea is protected by China. In Stalin's CCCP, even if Americans had wanted to lead an intervention, it would simply have been a spark that would have ignited a larger conflict that would probably kill as many civilians as Stalin's atrocities would anyway. You can see why nobody entertains these ideas.

For reference, Stalin's economic and domestic policies likely killed roughly 10 million people whilst the civilian death toll from WWII in the CCCP was 13 million - 11 million if we subtract the 2 million that died due to German camp conditions. This is already larger than the atrocity dead, and speaks nothing to the losses the interventionalists would incur in the violence.

Arguably an intervention in the CCCP may have been less violent than WWII, but it would likely not be so much less violent that the sum Russian dead would be lessened. And what does America gain? Interventionalism in a 'modern' sense is always politically motivated - that is, the interveners get something out of it. Not so with stopping Stalin.

Sources:

Michael Cloderfeld, Warfare and Armed Conflicts – A Statistical Reference to Casualty and Other Figures, p. 465

When Hitler came to power and started his "concentration camps" they were originally for political prisoners, mainly accused socialist and communists. This took place in March of 1933 at Dachau. Political prisoners were treated harshly and murdered from the very beginning of the establishment of Dachau.

In 1936 the Nuremberg Laws were passed that heavily discriminated against people of Jewish ancestry. But it wasnt until 1938 and the Night of the Broken Glass that Jews were rounded up and sent to concentration camps. These camps were not initially intended as wholesale mass murder camps but most of the world knew that a lot of people who went there died of brutality, disease and starvation. Yet WWII only started when Germany invaded the sovereign nation of Poland. WWII did not start because of other nations interfering in Hitlers plans to exterminate political prisoners or people of Jewish decent.

Stalin too, started his murderous regime in the 1930's, with attacks on political opponents and people deemed "enemies of the people and socially harmful elements.". When the Soviet union invaded Poland (with Germany) the Brittish did not see any advantage in declairing war against Russia, but did declair war against Germany in part because of a pact with Poland via the 1939 Angalo-Polish Treaty but also because Germany had proven itself a threat to Great Brittan and France, not because they were murdering their own citizens. And the US didnt jump in the war until December 7, 1941 after being attacked by Japan.

In conclusion I think your premise is wrong. I'm not saying the US has never interceded on behalf of the citizens of a sovereign nation, but its rare that is the only reason. Usually its a direct or indirect threat to other nations.