You have to keep in mind that Parris is the religious leader of the community. He is the one to guide everyone else in Puritan life and belief. If his household is tainted with corruption it undercuts everything he preaches to the rest of the community. Financially he is supported by the community. If they were to lose faith in him they would stop their financial support and Parris and his family “would be out on the street.” That is why it was imperative for him to place the blame elsewhere. Witchcraft seemed to be the logical conclusion because it was something that was difficult to prove as well as disprove.
In his opening notes to the play, Arthur Miller explains that before the witch trials begin, the town of Salem was already involved in a great deal of infighting. Parris's appointment as the town minister had not set well with many of the townspeople. They had had another candidate in mind, and so from Parris's first day on the job, he was on the defensive.
In addition to the original opposition to Parris's appointment, Parris was overly concerned about what others thought about him. He reminds the townspeople that he is a Harvard graduate and his experience as a businessman in Barbados most likely increased his desire for the "finer" things in life--again, something that did not set well with many of the parishioners.
One last note: In early American days, because of the small, scattered population, men of learning built their careers by gaining appointments in one town and then working their way into larger churches and towns. Parris, no doubt, had intentions of using his position in Salem as a stepping stone to further his career. Miller includes this information about Parris's character, and the judges' attitudes for that matter, to shed light on the religious hypocrisy and greed which led to the witch hunt.
What characters were concerned with their reputation in the Crucible