Why did the evacuees leave home?

Because nobody was expected to drop bombs on the countryside.

From the beginning iof the war, it was expected that Germany would bomb all major cities. The whole country was zoned according to its presumed danger; so London was a place of danger, and children could be sent out for their protection; Devon was a place opf safety, so households could be made to accomodate childre.

This led to one or two curious anomalies. In one North-Eastern area, a port city was directly next to a holiday resort. The city was a place of danger, the resort one of safety. One street ran along the boundary, and in theory a child could be evacuated from one side of the street to the other.