With four or five strings, you can play each note at a multitude of positions. With a more or less sensible choice of where you play each note, you can minimize movements of your left hand up or down the neck. With two strings you can still play all notes but you'll have to jump around a lot more. By building your bass line around the fact that you have just two strings you would get away with it without too many negative consequences. With a guitar where you play chords it would be a totally different story, but just look how many of the oriental stringed instruments that are played note by note have just two or three strings.
