Why is that only Nazi Germany is condemned for invading Poland while the Soviet Union also annexed a large portion of Poland and committed nearly half a million troops?

I assume that the OP is referring to the 1939 invasion of Poland.

The short answer is that a good deal of global public opinion labeled the Germans as the aggressors for acting first. The Soviets portrayed their invasion and subsequent annexation as a return of a lost territory (the Western Ukraine and Western Belarus) or a proactive protection of the region's peasantry from the exploitative landlords (Soviet propaganda often mixed the two rationales).

The Soviet's public image paid dearly for this invasion. Despite the presence of a few true believers abroad, a good many Communists and leftists outside the USSR saw the Molotov-Ribbentrop pact as an ideological betrayal. The Comintern tried to propagate the rationale behind the Soviet invasion, but the damage was done. The organized left either became estranged from the USSR or ideologically incoherent in their defense of Stalin's actions. The Soviets would not regain this moral high ground until Barbarossa as the Nazi's portrayed the invasion as an ideological crusade against racial inferiors.

Ultimately, as brutal and exploitative as the Soviet regime was, it never approached the scale of the Third Reich's genocidal actions. Although Stalin had carved up Poland, there was enough of an ideological gloss to Stalin's mass deportations and annexations to differentiate Stalin from Hitler. The Soviets never explicitly stated that the complete dissolution of Poland was their endgame. Moreover, the Soviets constrained postwar Polish memory to only commemorate German atrocities and subordinate them to Soviet victimhood. Although Katyn and other Soviet atrocities remained in Polish collective memories, there could be no real official commemoration of them within Poland. Outside of Poland, this commemoration often got subsumed under anti-communism within the context of the Cold War.

Sources

Furet, François. The Passing of an Illusion: The Idea of Communism in the Twentieth Century. Chicago [u.a.]: Univ. of Chicago Press, 1999.

Geyer, Michael, and Sheila Fitzpatrick. Beyond Totalitarianism: Stalinism and Nazism Compared. New York, N.Y.: Cambridge University Press, 2009.

Snyder, Timothy. Bloodlands: Europe between Hitler and Stalin. New York: Basic Books, 2010.