Because Poland was not a part of the Soviet Union, whereas the Commonwealth of Independent States comprises former Soviet republics. You might be confusing countries in the Soviet sphere of influence, also called "satellite states", with actual members of the Soviet Union. Eastern European countries such as Poland, but also Czechoslovakia, German Democratic Republic, Romania and others were indeed mostly puppets of Moscow, but also nominally independent countries. While the USSR was a centralized union of several entities like the Russian, Byelorussian or Ukrainian soviet socialist republics. An exception of sorts are the Baltic countries which, unlike the rest of European states under Soviet control, were annexed to the Union directly during the war.

u/CCCVCCCC 's answer is correct. I just wanted to add, though, that the C.I.S. is an organisation with a defined membership, so it's not a matter of who is or isn't "considered" a member of the C.I.S, but who actually is a member. It's not some nebulous group whose extent varies according to perspective, like "European" countries or "Western" countries or "First World" countries, but a concrete group with a defined membership based on application and acceptance.

The C.I.S. isn't made up of all former Soviet countries - some were offered membership and turned it down (namely, the Baltic states) while some left because of problems with the organisation (Georgia).