It would have complicated the design to make the arm reach backward. Besides, nobody knows the nature of the dust. A duster might make the problem worse by compressing it. Besides, they figured other parts would have worn out as well. No use fixing one part without consider the others. And the rovers do have joint and wheel problems. They just have not been severe enough to prevent further exploration.

Now that we know about the dust devil effects, a compressed air blower may be the way to go since wind has proven effective.