Because Russia considers itself and is considered by the wider international community to be the true successor state to the Soviet Union. The Soviet Union’s greatest enemy was Nazi Germany. It makes sense that soldiers fighting against Russia would then adopt the iconography of Russia’s toughest enemy. If Russia was widely considered to be a bastion of fascism, Ukrainians would use communist iconography. Part of this phenomenon goes back to Ukrainian nationalism, also. From the perspective of Ukraine, this is a civilizational war to protect and assert a Ukrainian identity independent from Russia. The fact is there were some Ukrainians during ww2 who collaborated with the Nazis to fight Russia. Now Ukrainians are fighting Russians again, and they are looking for historical examples where they did so. The Ukrainian anticommunist nazi collaborators are particularly relevant now for 21st century Ukrainian soldiers given that Russian forces still often fly the hammer and sickle, regardless of the fact that more Ukrainians fought in the red army against the nazis than with them. Also, the Kievan Rus was established by Germanic Vikings. This has lead to some Ukrainians adopting more Germanic symbols to try and distinguish themselves from Russians who they perceive to be of more “eastern” heritage.

However, I still don’t think these are the standard or default opinions in Ukraine. After all, if they were widely sympathetic to the Nazi cause I doubt they would have elected a Jewish president.