Pope Called For a "Holy War"

When Pope Urban II called for a Holy Crusade in 1095, it was at the request of Alexios I Komnenos to help defend the Byzantine Empire against the invading Seljuq Turks. It was supposed to have been a military exercise, but it became known as the First Crusade, a "holy war" with the Christian objective of winning back the Holy Land from Islam. Why the sudden interest in Jerusalem after nearly 500 years?