It was a few factors, in addition to Stalin being in power for more than over a decade prior and the higher death count, Stalin also had first cultivated a cult of personality, purged not only critics but allies too, and though Stalin did it in the name of communism's idea of atheism, Stalin and Hitler were both personally antisemitic and targetted Jews. They both targetted Catholics and other religious and ethnic groups for example Slavs for Hitler and Ukrainians for Stalin, both targeted Gypsyies just to name 2 groups. And although Stalin did allow for the Russian Orthodox Church to start operating again openly in Russia during WWII and to continue (Stalin prior to joining Lenin's communist group had been a seminarian and was training to be an Orthodox Church Priest) because it was his religion and he allowed for a worship of it because it was sanctioned by him, while Hitler was building a religion that was based on his vision that would be sanctioned by him. The OSS (the CIAs predecessor) had a lot of knowledge on these men that was shared between themselves and the British and they saw little difference between Stalin and Hitler. Also some at that time actually considered Stalin worse and that working with him after Hitler turned on Stalin in Barbarossa as a kind of a deal with the Devil.