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/