Because the Old City of Jerusalem is where the holiest places for all 3 faiths - Judaism, Islam and Christianity can be found. It’s also the place of the 4 main quarters - Muslim, Christian, Jewish and Armenian. For Jews, the Temple Mount and the Western Wall (also called the Wailing Wall) are the holiest places, for Muslims it is the Dome of the Rock and the Al-Aqsa mosque and for the Christians it is the Church of the Holy Sepulchre, one of the major pilgrimage sites.