I mean, one of the main reasons people wanted to leave Earth was because of the war. Then why did they go back to Earth when the war started? I understand that people cared for their families and friends, but even so, in real life most people won't return to a war zone after they escaped.
This book was written soon after WWII and at the time there was a patriotic atmosphere where everybody wanted to enlist in the army. So maybe his thought was that everyone would feel compelled to go back to earth to defend their country.