Because this is the absolute cornerstone of russophobia and anticommunism in Poland. If that fall, entire construct will get very heavy blow. This is the reason it is completely impossible to hear any conflicting message in Poland, and anyone who is trying to dig deeper than official propaganda is immediately labeled “stalinist” “conspiracy theorist” “insane” and even “traitor”.

For comparison: in official polish lore even things like Volhynia massacre and Holocaust are forgiven now. But Katyn… never. Also, it’s not even the only one thing falsely attributed to the Soviets, there is also the intelligentsia extermination carried by nazis, which is rarily even mentioned now, and Soviets gets accused of it, with some time-space spin apparently.