In Act I, what explanation does Miller give as to why the witch hunts developed in such a community in The Crucible?

Miller's extensive stage directions suggest several
reasons why the Witch Trials had to take place in Salem.  Miller argues that the
fundamental nature of Salem's construction made it a community where the Witch Trials
were inevitable.  The "parochial snobbery" as well as a "predilection for minding other
people's businesses" helped to make Salem a prime place for the trials to emerge and the
charges of witchcraft to emerge on such a wide scale. On a more material level, the fact
that the land charters to Salem had been revoked helped to create an air of tension
about land ownership.  This helped to feed the paranoia that people felt about one
another.

Perhaps the most intense reason why Salem had to
be the birthplace for the witch trials resided in the idea of the authenticity and self-
certainty that gripped Salem.  Miller points out that this is one of the most important
reasons why the trials took place:

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... They [residents of Salem] carried about an
innate resistance, even of persecution. Their father had, of course, been persecuted in
England.  So now they and their church found it necessary to deny any other sect its
freedom... They believed in short that they held in their steady hands the candle that
would light the world.

This
tendency to believe in the certainty of one's convictions as well as the belief that
their practices of exclusion were justified among the cultural conditions of Salem. 
These stage directions allowed the reader to gain insight as to why Salem was able to
serve as home to the witch hunts.