Why do the people leave the city and flee for their life in this passage?
They leave the city because of their physical fear of what is just taking place. As refugees have always done, they flee for their lives. Not all do so, but a remnant departs. The passage later pictures the woman, Israel, fleeing into a wilderness place prepared by God for 1,260 days.
Les explains that because of their physical fear of what is just taking place, they do what refugees have always done: they leave the city and flee for their life, though not all—only a remnant. He ties this to the symbolism where the woman is the nation of Israel, who brought forth the man-child, the Christ child. At the midpoint of the tribulation, the woman flees into the wilderness, the mountains Jesus spoke of, where she has a place prepared of God, and the Godhead feeds her there for 1,260 days, or three and a half years.