The points at which the game transition between acts seem a bit arbitrary (mainly for Act I to Act II), and I don’t see a narrative or mechanical reason to lock us out of previous maps and quests. As far as I remember, previous Baldur’s Gate games didn’t have this kind of points of no return. Why do you think they did it? Do you like it?
You can still travel between act 2 and act 1, I think the quests are locked out because the NPCs involved have moved on to the new area. It’s a little annoying to be unable to finish some things up but I don’t really mind overall. It’s nice that the side characters make some progress in the story.