Why did the First Crusade prevail?

"Most Crusades historians now believe that the First Crusade prevailed because the Arab world was already disintegrating by the late 11th century. In the first three centuries after Muhammad's death, Islamic and Arab civilization were synonymous. The Arabs dominated an area that encompassed most of Asia Minor, North Africa, Spain, and Southern Europe. Aside from the Byzantines, they maintained the most highly developed civilization in Europe and western Asia at that time. This is not the complete story, however.

Prologue of the Crusades: Jerusalem and the Seljuks

"When the Muslim Seljuks, a Turkish steppe tribe, spectacularly defeated an army of the Byzantine Empire at the Battle of Manzikert in ancient Armenia in August 1071 CE, a series of events followed which would lead to centuries of East-West warfare couched in religious terms: the Crusades.