I just wanna preface I am not trying to downplay what the Germans did. But what I don't understand is why Germany had the allies declare war on them for invading Poland, but the soviet union invaded Poland with the Germans, and would also invade Finland, but war was not declared on them. In addition to this invasion, similar to the Germans, the soviets would round up and exterminate any poles that they saw as a political threat to their control over Eastern poland. So why didn't the allies declare war on the soviets too, and why weren't the soviets atrocities towards the Poles called out during the end of the war?
As for the first part, because the secret protocol of the Polish-British Common Defense Pact stated in no uncertain terms that the meaning "European power" in "Britain would come to the aid of Poland were it to be attacked by a European power" refered explicitly to Germany and Germany alone. (More on that here).
As for the second part, pragmatism. When the massacre of Katyn became known to the allies in 1943 they figured out almsot immediately that the soviets had to have been responsible for it, as did the Polish government in exile. At the time it was judged that a continued alliance with the USSR was crucial to the war effort and as such the information was suppressed, a decision that further strained the relations between the Polish goverment in exile and their British hosts.
The Polish goverment called for a Red Cross investigation of the Katyn mass graves. Something Stalin flatly refused. As did Germany, something the USSR presented as proof of a Polish-German conspiracy. Churchill asked Owen O'Malley (British Ambassador to the Polish goverment in exile) to investigate the issue. O'Malley rather quickly concluded that the massacre had been carried out by the NKVD and that the Soviet version was full of inconsistencies. The British government decided to keep this information secret but the O'Malley report was nonetheless forwarded to Roosevelt. Roosevelt in turn asked George Earle (At the time Special Emissary to the Balkans, previously governor of PA) to investigate the issue. Earle, like O'Malley concluded that the massacre had been carried out by the NKVD to which Roosevelt basically nodded and stuck to the official narrative that the Germans were to blame. This upset Earle who wanted to make the information public. The United States Office of War Information kindly suggested he think about that for a while, perhaps while on vacation in Samoa. For say... the remainder of the war. Earle did.
At the Nürnberg trials the soviet delegations tried to include the Katyn massacre among the war crimes of Nazi Germany but failed to adequately prove it and as such saw it dismissed. During the following Cold War it was generally kept quiet to avoid needlessly antagonizing the Soviets and it was not until the late 80's that it was finally recognized.