First, because the historic view is postmillenial - it is what drove the Pilgrims here to begin the “New Jerusalem.”
Second, because the church was weakened dramatically and lost its influence in culture since the date setting gained prevalence.
Dispensationalism has been traced all the way back to the 1st century.
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I still don’t understand why “Christians” would propound a clearly false theory as fact.
Hair-brained rationalizations just don’t cut it for me.
It sounds like you are telling me that social forces decided truth for a bunch of people.