Earlier quoted context omitted.
That's because there has not been a disruptive change in USD banknotes for a very long time. And it's desperately needed -- their cotton paper notes are just crap. The Australian switch to polymer meant an entirely new suite of banknotes which bear almost no resemblance to the paper ones they replaced.
At this point there's a decent installed base of automated bill-reading machinery, though. Not that these ought to be sacrosanct or anything, but the cost and inconveniences associated with a rollout of new bills is substantial. Plus, people will complain about the new ones. :)
The classic example is the penny. Get rid of it! You can still collect them and read about them on wikipedia, you just won't have to deal with them while buying your double cheeseburgers any more.