That's not what happened, and you know it. The Palestinians mostly fled during the 48 Arab-Israeli war. Some were undoubtedly killed or expelled, others fled. Those who have been able to prove expulsion have actually been compensated. They fled because of Arab propaganda that the Israeli were tourturing and killing all Muslims they could find, something that was clearly untrue considering that the ones that did stay still live healthily in Israel today.

Since the people who fled were part of the war against Israel, why should they have let them back in? "Oh, sorry about you trying that war against us and all, please come back in and try to murder us again!".