Why did the Byzantine Emperor call for help from Pope Urban II?

The Byzantine Empire was considerably overstretched. As such, it was difficult for the Emperor Alexius I to defend his vast borders from attack. Many forces threatened the territorial integrity of the Byzantine Empire, but the biggest single threat came from the Seljuk Turks, who made increasingly bold attempts to break through the empire's eastern border. By 1095, the situation was dire; the Turks were threatening to attack the Byzantine Empire and take its capital city, Constantinople.
Faced with such imminent danger, Alexius felt he had no choice but to make a personal appeal to Pope Urban II for assistance. Relations between Rome and Constantinople had been strained for some time. The Great Schism of 1054 had led to an irreparable breach between Eastern and Western Christianity, one that still exists to this day. But desperate times call for desperate measures, and so Alexius made his urgent appeal to the pope. For his part, Urban welcomed Alexius's plea as he saw a golden opportunity to unite Christian Europe under him and greatly increase the power and prestige of the papacy as a consequence. Ultimately, this led to the First Crusade.