Why did Pope Innocent III excommunicate the Venetians during the Fourth Crusade?

Pope Innocent III excommunicated the Venetians because they diverted the Fourth Crusade from its original objective of liberating the Holy Land to attack the Christian city of Zadar (Zara) in 1202.12

The conflict arose after the crusading army failed to pay the full amount contracted to Venice for transport and provisions. To recover their costs, the Venetians, led by Doge Enrico Dandolo, persuaded the crusaders to capture Zadar, a city that had rebelled against Venetian rule. Despite receiving explicit letters from the Pope forbidding the attack on a Christian city, the army proceeded with the siege.12

While Innocent III later granted conditional absolution to the crusaders who were under financial duress, he refused to extend this to the Venetians, who were the primary instigators of the diversion. The Pope’s opposition continued as the Venetians later pushed for the further diversion of the Crusade to Constantinople, an action he also prohibited.2