"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.
Beginning in the 11th century a series of invasions from Central Asia began to wear down the borders of both the Byzantine and Islamic Empires. The Byzantines retained their unity, while watching their land and influence shrink. The Arabs, meanwhile had already split into several competing nations -usually along the lines of Sunnism and Shi'ism.
Arab Shi'ites and Sunnis divided during the war of succession following Muhammad's death. The Shi'ites favored a hereditary line of succession in Muhammad's nephew and son-in-law Ali (and his son, Husayn) as Caliph, spiritual head of the Islamic faith. The Sunni faction favored succession by election. Ali initially became Caliph. Sunni forces managed to murder both him and Husayn -but Husayn left descendants. The Sunnis won the initial battle, but, so far, had yet to win the war.
The Arab civilization fragmented under the assault of the barbarian invasions. The first wave of invaders were called the Seljuk Turks. They swept into Asia Minor where they were bought off and converted -much as the Normans' Viking ancestors had been in 10th century France. As the Turks settled in, they increasingly pushed local Arabs out of positions of authority.
Muslim historians had difficulty explaining this humiliating conundrum. The Arabs were being pushed out of their own civilization by newly-converted Muslims. To make matters worse, while Spain, North Africa, Palestine, and Syria were predominantly Shi'ite, the Seljuks were Sunnis. Around 1090 -nine years before the Crusaders took Jerusalem- the Holy Land went from Shi'ite to Sunni domination. Needless to say, this created considerable resentment among the population, which could not have entirely converted in so few years. This resentment led to protest movements such as the Assassins, who were an lsma'ili sect of Shi'ites.
Because of this intra-religious antipathy, and a loss of interest in the concept of jihad, and expansion, after the 9th century, the Muslims in Palestine were unable to organize effectively against the Christians."
This blog quotes, with a minor addition on the sunni-shi'i relationship, sections of pp 90-91 of Stiles, Paula Regina, "BETWEEN TWO FAITHS: THE ARABIZATION OF THE KNIGHTS TEMPLAR DURING THE CRUSADES" (1999). Open Access Master's Theses. Paper 1805. The illustration shows the Destruction of the Tomb of Husayn ibn Ali at Karbala, condemned in a Mughal era manuscript, created aboput 1590, Public Domain, source Wikipedia