Because this would require genuine effort and interest on the developers part as they would have to learn a lot of lore and backgrounds to FR/D&D and discover concepts such as narrative cohesion which where not only completely absent in their previous titles but they didn't even consider something as basic as narrative continuity.
No need to throw stones now. D:OS (from what little I've seen) has a very different tone to the type of high quality narrative that players expect from Baldur's Gate. They expect the companions to feel like living, breathing people that are just as invested in the story as you are. That's a high bar, and it may take several passes for Larian to get it right. It's stilly to think they didn't put in effort or don't have interest.
