There was a war that collective Arab nations started, and those who left were given an opportunity —–The United Nations General Assembly Resolution 194 is a resolution adopted near the end of the 1947–1949 Palestine war. The Resolution defines principles for reaching a final settlement and returning Palestine refugees.
UNRWA definition of a Palestine refugee in 1952: “A Palestine refugee is [a person] whose normal residence was Palestine for a [minimum period of two years] preceding the outbreak of the conflict in 1948 and who, as a result of this conflict has lost both his home and his means of livelihood.” –In the 1948 General Assembly proposal included all stateless persons who had been residents of Palestine for a minimum of two years.