Not really surprising from a sequel to a niche old school title. I think it's the same thing as DLC selling poorly in comparison to main game, only a fraction of the people who bought the first game transition to the next one, and those who haven't yet played the first probably aren't even considering the second.
As much as I have love for the old school cRPG genre, I still haven't gotten around to actually playing through any of the new titles, and the latest one I did play was Siege of Dragonspear. That's likely due to BG being nostalgia comfort food and doing the expansion in the middle was easier than to jump in a totally new game that might take 100+ hours to complete. Since there's a glut of great games anyways, these titles have dropped to the "play some day" category for me, especially since they keep getting more updates and content, and getting a package deal later on just makes more sense because I wouldn't play them right now anyways, the commitment is just too big.
Overall there's just very little reason to play them at launch and few people will actually do that, which obviously has implications for profitability. I'm glad at least Larian is doing well enough to keep at it, but I don't think this market segment is going to support more than that. It's pretty much up to charity of those feeling nostalgic for cRPGs to maintain their development. I think modernizing the genre in some aspects may make it more palatable for a larger audience, and I think D:OS did that in some aspects. The problem with that is increasing dev costs and uncertainty of whether it'll pay for itself. That said, I think the days of making 2000's looking "modern" cRPGs are pretty much done. You have to do better than that to sell copies.
