The following paragraph from this Le Monde article seems to explain their reasoning:

Zelensky's changing stance on these memory issues comes as the Kremlin's invasion of Ukraine in February 2022 has profoundly altered Ukrainians' relationship with their history. The authorities are now working to create a national pantheon to honor multiple figures of Ukrainian nationalism spanning centuries. Zelensky thus finds himself in a delicate position, caught between the risk of not meeting the demands of a narrative centered on resisting Moscow and the international repercussions of such memory policies.

Because the unit fought both Nazis and Soviets to try to carve out an independent Ukraine. The attack on the poles wasn't the "main" thing they did. It was also an insurgency unit not a regular unit so it's hard to treat them as one thing. The only people who consider it the main thing are the Poles.

Here is the Polish deputy Science minister on the matter and probably one of the few rational voices in the debate

It was a unit that — regardless of what you say about the Volhynia massacre — fought for Ukraine’s independence,” Szeptycki said in a radio interview, and compared the UPA to Poland’s anti-communist guerrillas after the end of World War II.

https://www.politico.eu/article/volodymyr-zelenskyy-poland-world-war-ii-ukrainian-military-unit-polish-politics/