Most historians argue that there were calls for religious revivals during this time because Americans in general were becoming less fervent in their religious belief.
During this time, the ideas of the Enlightenment had become quite strong in the American colonies. These ideas tended to push people away from traditional religious belief and towards rationalism. In addition, there were fears among many that too many people on the frontiers of the colonies were out of touch with religion because of a scarcity of churches.
There were two major reasons for these calls, then. There was a fear that the rationalist ideas of the Enlightenment were leading people away from piety and there was a fear that the people on the frontier were lapsing into sin because they lacked churches.