The initial plan wasn’t to give the entire area to the Jews, it was to give some share of it (20% of the land, is the figure I heard). That area is the only place the Jews could really conceivably lay claim to. And the Arabs (specifically the Sharif of Mecca, not the people of Palestine) got huge swaths of land in exchange for their revolt against the Ottomans: Saudi Arabia, Jordan, Lebanon, Syria, etc. The British made a specific exception for coastal areas, and there’s debate about whether Palestine was part of that or not.

So…not that simple.