The Temple Mount is the holiest place to the Jewish people for the last 3,000 years. Beforehand, it was unknown as a Holy place, but there was Jewish history there for more than a thousand years beforehand too. But since the time of King David, Jerusalem has been the center of Jewish national existence, and the Temple Mount has been the epicenter of that existence.
The Temple Mount was the location of the two Temples, each that stood for more than 400 years. But for nearly 2,000 years, the Temple Mount was under foreign domination by all of the foreign countries who dominated the Land of Israel. The last foreign force before the formation of the State of Israel was the British Empire. Jewish people struggled to pray at the Western Wall, outside of the Temple Mount. But, they did everything they could in order to keep their connection with the remnant of the Temple.
What changed was that in 1949, at the end of the War of Independence, Israel found itself distanced from the Old City, despite winning the war. The battle for the Old City ended in defeat to the Jordanian Army. For 19 long years, Jews were not allowed near the Western Wall or the Temple Mount. When Israel retook the Holiest site in 1967, it made the fateful mistake of handing back the keys to the Temple Mount to the Arab Waqf. This is what led to so many of the problems there are today.