There are several reasons. Parris is closely tied to the courts and all that they do, and the general feeling of the townspeople has changed quite a bit since the accusations started. Where before the town fully supported-and even promoted-the accusations and hangings, the townsfolk are now, after all of the arrests and hangings, starting to rethink what is going on. In addition, Parris is worried that "the rumor here speaks of rebellion in Andover", which indicates that the people of the neighboring town of Andover have overthrown the courts. Danforth tries to deny it by saying "Andover is remedied. The court returns there on Friday" but Parris is not convinced. Parris thinks that Abby "had close knowledge of [Andover]" and that she was worried that rebellion would occur in Salem too, so she bolted, for her own safety. Since Parris is closely connected to the courts and accusations, he is worried that one more hanging might tip the townspeople over the edge, turning against him.
Salem is in fact turning; Parris found a dagger stuck in his door, indicating there are ill feelings towards him and the courts. To add to this, the cows are wandering, fields are unplowed, and children are orphaned. All of these factors-the turning public opinion, Andover, the devastation left in the wake of the trials, and Parris's close ties to the court, leave him worried for his own personal safety. So, postponing the hangings might stem that tide a bit.