Evelyn Gordon puts it so succinctly.
Though there had long been a lively debate as to whether Israel ought to hold on to these territories in practice, until 1993 all sides were ready to assert that it had a valid claim to them in principle. The argument in favor of Israel’s right to sovereignty there was simple: these territories are the historic Jewish homeland, the heart of the biblical Jewish kingdom. They were explicitly allotted to the future Jewish state by the 1922 League of Nations Mandate, which was never legally superseded. Although the 1947 UN partition plan allotted part of the land to a putative Arab state—a plan that Palestinians and other Arabs rejected as a matter of principle—it was merely a nonbinding “recommendation” (as its own language stated). Thus once the Arabs rejected it, the measure had no more validity than any other unsigned deal. Nor did any sovereign state ever replace the Mandate on this territory: though Jordan and Egypt conquered the West Bank and Gaza, respectively, in 1948, neither conquest was ever internationally recognized. Legally, therefore, the territories remained stateless lands whose ownership is disputed; over time, the Palestinians simply replaced Egypt and Jordan as the Arab claimants.