UNRWA was set up especially for the Palestinians because unlike other refugee situations, the Palestinian refugees were as a result of actions (or more exactly inaction) by the League of Nations and its heir the UN.
The UN failed to insure that pursuant to the Palestine Mandate that:
"nothing should be done which might prejudice the civil and religious rights of existing non-Jewish communities in Palestine..."
In fact, both the civil and religious rights of the existing population of Palestine were prejudiced by the UN by its partition of Palestine, leaving a third of the Palestinian people within a partition to be ruled by Jews.