The Turks were defeated by Crusaders at Jerusalem because

The Turks were defeated by Crusaders at Jerusalem because "they could not remain united," since it was this lack of unification that led to their forces being divided and reckless.

Because their forces weren't as prepared, as motivated, and as eager to win that battle. The Crusaders were also employing better tactics at the time. All of these things lead to a subsequent difficulty for the Turks of hoping to achieve victory.