In The Crucible, why is that time the first time they accuse people of being witches?
In actuality the idea of witches and witchcraft goes back as far as recorded history. Every civilization has had its view of those who appeal to the forces of nature for aid and assistance. Since our roots are in Western culture, our concept of witchcraft traces back to the old English views of "wicce" (witch) "crafte" (practice).
If the question is about the presence and practice of witchcraft in the Americas, and specifically the North American continent, it is also not accurate to assert that the events recorded in the Salem Witch Trials were the first accusations on the continent in modern times.
The Crucible is set in the years 1692-1693, yet fifty years earlier in Springfield, Massachusetts there were accusations of witchcraft when Hugh and Mary Parker, husband and wife, accused each other of being witches concerning events surrounding the death of one of their children. Both were acquitted, but Mary died in prison over other charges springing from the death.