Both were parts of the former Russian Empire, and both were occupied at the end of the war. So why one and not the other? I'd be interested to learn both the stated rationale and the likely actual reasons, if they differ.
Nazi Germany's invasion of Poland was the casus belli for Great Britain and France to declare war on the Nazi regime. In a sense, Poland's freedom was what the British and French fought for. So when fixing borders in the post war settlement, annexation of Poland by the Soviet Union was unpalatable for the British and French. Stalin was aware of this consideration.
The Soviet Union pushed for the development of People's Democracies throughout their zone of influence in Eastern Europe. This form of control did not violate the Yalta principles calling for democratic self determination in European nations liberated from Nazi control. There was a veneer of democratic institutions in these People's Republics, but within this zone, military cooperation and foreign policy alignment with Moscow were enforced.
So Stalin did not annex Poland because that outcome was unacceptable to his allies in WWII. He settled instead for a Polish satellite state/client republic which would lean Communist and provide a buffer between the Western capitalist democracies and the USSR.
Debating the Origins of the Cold War | Levering et al | Rowan and Littlefield 2002