A relatively easy answer to this is that the Soviet Union wanted to look on the up-and-up when it came to its handling of Poland at the end of the war / post war. Poland was the entire reason WWII was started in Europe. The Soviets invaded Poland two weeks after the Germans did, and they annexed and and seceded Eastern and Southern Portions of Poland to the Ukrainian Soviet Socialist Republic (such as Lvov / Lviv) and the Belorussian Soviet Socialist Republic. After Poland was liberated in 1944/1945, the Soviet Union never gave back the Polish territories they had annexed and incorporated at the beginning of the war.

The Western Allies had a vested interest in the fait of Poland, but an even stronger interest in maintaining peaceful diplomacy with the Soviet Union, so as not to start World War III right on the heals of WWII. Poland would fall in to the Soviet sphere of influence, as would the newly reformed Czechoslovakia. Their doctrines would be Communist (even though the Armia Krajowa [Home Army] was the largest non-Communist Resistance Movement during the War, and most Poles did not want a Communist / puppet Soviet rule). But since the Soviet Union had taken Polish territory and diminished its pre-War size, and since Germany had large tracts of land that were historically Polish, and expanded upon during the War (Silesia, Pomerania, the Danzig Area, parts of East Prussia etc...) the USSR felt the best way to appease the Western Allies (and the Poles, especially located in areas that were now part of the USSR) was to strip Germany of essentially all of East Prussia (with the USSR taking the Konigsberg capital region and making it a little Russian inland island in the Baltic states), Silesia, Pomerania and the Danzig / Vistula mouth area and give it to the Poles. The main German border was pushed westward to the River Oder. Poland's borders were thus shifted Westward to incorporate these new Polish territories to make up for the regions they had lost in 1939 to the USSR.

The ethnically Polish minorities still left in those areas, especially say Silesia, were given control. The Soviets resettled Poles from the former eastern parts of Poland (such as Lvov / Lviv) in the newly acquired territory (so ironically, Western Poland was no ethnically largely made up of pre-war Eastern Poles). The German civilians and former military personnel still in those areas, even in large cities like Breslau, were forcibly deported in the Fall of 1945 through Summer of 1946, with the winter of 45/46 being especially hellish. The vast majority were loaded in to cattle cars by Soviet occupation authorities and shipped Westward / North West in to East Germany. Some lucky ones managed to make it to West Germany though. But most were resettled in the DDR. The Poles in such places treated the ethnic Germans horribly, as a kind of retribution for their treatment by the Nazis (and most likely by the Soviets) during the War, and it was in fact the Soviet occupation forces that had to kind of keep some order and discipline in the years after the war.

Battleground Prussia, Savage Continent, Fortress Breslau, and even A Higher Call (book about an American and German airmens heroic encounter, with the German being from Breslau and living there after the war for a time) provided much of my knowledge on this subject.

Also to TL;DR my response, the USSR stole Polish territory, it wanted to look good in the eyes of the world, so gave Poland German territory to make up for it. The USSR did not take Czech territory.