Because then SE would have to make 2 storylines for the main story...which would only result in 2 half baked storylines, rather than one well done storyline.

Pretty much like all the western RPGs(don't get me wrong I like most of them), but the story is always half baked cause the devs give you the "freedom" to choose many different paths, which is a nice thing to be more individual...but the different paths are always only half as good as a really well focused main story for one specific path could be.