Fans would be a smarter solution. Wiper blades plus dust and dirt = a mess, not necessarily clean solar panels. Engineering something that would be reliable on Mars would be rather difficult, and would add mass and complexity to the rovers, which would take away from the mass needed for other instruments and time needed for testing everything else. Also, dust buildup on the solar panels was only one factor limiting the expected lifetime of the rovers, so instead they chose to balance the engineering of the vehicle and concentrate on hitting the base mission lifetime expectancy.