Because, like I said, they don't make many sights any more with pins that are individually adjustable for windage. Say you sight in at 40 yards with a quarter bubble cant to the left. Your pins will be off to the left at 30 yards and even further off at 20. Same for 50 and 60, but in the other direction.

If you want your arrows do go down the middle on each shot and have the same degree of cant to your bow each time, your sight pins cannot be parallel to the sight window. And a single pin sight is out of the question.