OK, so what do you suggest they use to make the rocks move? They had a low budget and it would have been too much trouble to go back and erase a piece of nearly invisible fishing line. It's rather difficult to make a rock move, especially when the Force is non existent.
Actually, after the first movie (A New Hope) was released and made MAJOR money, the producers were able to move to a bigger studio, had a larger budget, and had much more time to work on their movie. You should read the Insider more often!