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 aren’t told this in game, tho. The quests in question don’t even apply the pressure to give the illusion time is crucial, the way they do with the worm in your head every 5 minutes. You have no way knowing that these two specific quests have a time limit. It’s not like every quest has them.