I would say because she would not want to meet Chillingworth one on one. So, would that be right?
Symbolically the scaffold represents the strict moral code of the Puritans. It displays Hester's act and her punishment and the only place where Dimmesdale is safe from the reach of Chillingworth. It also represents acknowledgment of sin. It is here that both Hester and Dimmesdale acknowledge their sin.
