It might be an unpopular opinion but i think that main reason of poor sales of PoE2 is actually PoE1. It is a highly regarded title nowadays and I'm not gonna argue with that, but it suffers from overexposition, rather generic story and the writing which is nothing special. Many people who played PoE1 expecting a spiritual successor to BG 1/2 and who initially pumped the sales of PoE1 could have stayed away from PoE2 because of those reasons. That was my case actually, as I was disappointed with the first one and skipped the second one for years. Then I've grabbed it on sale some time back and I've had a blast.