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?
So long as you don’t progress past the Mountain Pass into the Shadowlands you can go back and do the Underdark no issue, and you can still go back and hit the underdark even after that (all the way until Act 3) though you will miss some quests that way.