There wasn't enough money to pay for those boats, so the Venetians made the crusaders a deal. Help us capture the rebellious city of Zara and we'll ferry you to Anatolia. This was a smidge problematic, crusading-wise, because Zara was a Christian city, but the crusaders agreed to help, resulting in the Pope excommunicating both them and the Venetians. Then, after the crusaders failed to take Zara and were still broke, a would-be Byzantine emperor named Alexius III promised the crusaders that he would pay them if they helped him out, so the excommunicated Catholic crusaders fought on behalf of the Orthodox Alexius, who soon became emperor in Constantinople. But it took Alexius a while to come up with the money he'd promised the crusaders, so they were waiting around in Constantinople, and then Alexius was suddenly dethroned by the awesomely named Morsophos, leaving the crusaders stuck in Constantinople with no money. Christian warriors couldn't very well sack the largest city in Christendom, could they? Well, it turns out they could, and boy did they.