Therefore it was not used during the PRL.
It was but by Polish exile emigration in the West, the opposition in Poland and other people rejecting the regime. We sang Rota with changed words in 1980s - Soviet instead of German - aż się rozleci w proch i pył, sowiecka zawierucha. I also read general Ander`s book about Katyń - he used Soviet, not radziecki.
But now Sowiecki seems to be far more common..
Because it is more patriotic to say so. :):)