The success of the First Crusade was not inevitable. Most military historians consider it remarkable given the odds. Several factors combined to make it possible.
Muslim disunity was the single biggest factor — The Seljuk Turks and Fatimid Egyptians were enemies. Kilij Arslan assumed the crusaders were just another Byzantine raid and was away fighting elsewhere when Nicaea fell. Different Muslim rulers (Mosul, Damascus, Aleppo, Egypt) could not agree to unite. The crusaders faced them piecemeal, not together.
Crusader military organisation and discipline — Unlike the People's Crusade, the main army had experienced military leaders and maintained discipline. Heavy cavalry charges (the massed charge of armoured knights) were devastating against Turkish cavalry, which relied on hit-and-run tactics. The crusaders also used siege engineering effectively.
Religious motivation created extraordinary endurance — Crusaders believed dying in battle guaranteed salvation. This made them willing to fight under conditions (starvation at Antioch, brutal heat) that might have broken a purely mercenary force. The Holy Lance's "discovery" demonstrates how religious belief sustained morale at critical moments.
Byzantine support (at least early on) — Alexios I provided guides, supplies, and naval support at Nicaea. Byzantine knowledge of Anatolian geography was essential in the early stages. (Relations deteriorated later as crusaders refused to return captured territory.)
= A "perfect storm" — None of these factors alone would have been sufficient. Muslim disunity without crusader military skill, or religious motivation without organisational leadership, would have failed. All four factors together produced the unlikely victory.