The reason is because it doesn't work. How do you walk forward continuously for a long stretch? Do you roll it to some position and keep it there? Do you keep rolling?
Yet you have two such analog sticks. But usually only one is used the way you describe, to keep walking continuously. The other is usually used to rotate a bit, to look around you. So it seems having an analog stick on the left of the controller and a track ball on the right would be nice.
The rollers for the tracking ball would collect a lot of dust. Could be one reason they opted out of it. Cleaning the rollers for the track ball every other week to get a smooth feeling would quickly become bothersome, contrary to the nice feeling of relaxation or thrill the console is supposed to give you.
Granted, it doesn't reach console levels of use on most days, but I only clean it once every month or so, and even then it's easy - Logitech has a simple system to pop the ball out and cleaning it is very easy and fast.
Using a trackball is definitely not a major chore.
