Don’t hate on my man Innocent III, he said he was against attacking Christians and excommunicated the crusader army for attacking Zara. The sore loser Alexios IV Angelos offered the crusader army too much to ignore (from Wikipedia):

There, Alexios IV offered to pay the entire debt owed to the Venetians, give 200,000 silver marks to the crusaders, 10,000 Byzantine professional troops for the Crusade, the maintenance of 500 knights in the Holy Land, the service of the Byzantine navy to transport the Crusader Army to Egypt, and the placement of the Eastern Orthodox Church under the authority of the Pope, if they would sail to Constantinople and topple the reigning emperor Alexios III Angelos, brother of Isaac II.

That fucker got ousted by the public one year later and didn’t deliver anything so he essentially scammed the whole crusade. The other parts of the army not simping for Venetian and Thessalonian rat plans just went straight to Acre instead.