It really comes down to simple mathematics. Think about the influence of every single meaningful choice on the permutations of possible paths the player can take. Limit the amount of choices or limit the impact the choices have on the world. The game tends to present lots of options when it comes to things with limited effects on the overall story but it does not even allow you to get along with the goblins if you help them (probably because siding with them would change most of act 2). Not having illusion of choice when it comes to decisions that could change the entire story requires lots of resources.
I really enjoyed the depth of dialogue in Planescape. Many players would find it boring. Trying to please the "adventure with complex story and lots of choices" and the "giant world with lots of interesting combat" crowds never worked perfectly.
