There are entire books dedicated to answering this question, but I'll do my best to be brief.

When the 4th Crusade was being planned, logistical and strategic concerns were the main focus. Crusades that had approached Jerusalem from the north had typically failed, due to the difficult geography of the region and the fact that it was heavily fortified. Approaching from the north typically meant getting past fortresses on mountainous terrain, which wasn't easy. This had been a major issue as recently as 1197, when the German Crusade failed without accomplishing anything. The plan was therefore to attack Jerusalem from the south by taking Alexandria and then marching north. This was, on paper, an excellent plan since it struck at the economic heartland of the enemy whilst circumventing most of the problematic forts that dotted the Holy Land.

This meant that the crusaders would need to get there by ship. A delegation was sent to Venice to negotiate the construction of a massive fleet to take 33,000 crusaders along with their equipment and horses. This was... ambitious. There was no binding agreement between the crusaders to leave via Venice, and it seems to have been poorly communicated. A large number of crusaders went via smaller fleets from places like Marseilles and Genoa, and departed straight for the Holy Land rather than Alexandria since the leadership had chosen not to tell people about the Alexandria plan - it may not surprise you to know that the people in charge of the 4th Crusade had very little experience of large scale warfare.

Therefore, when the time came to pay the Venetians for their fleet (which they had already built), there wasn't enough money. The crusaders agreed to attack Zara, formerly a Venetian town, and split the loot to pay the debt. Even that wasn't enough.

At the same time, Alexius, the son of the former Byzantine emperor, arrived in Italy. He sought out the crusaders (and was possibly put into contact with them by Philip of Swabia) and made them one hell of a deal. He would pay off all the debt with a grant of 200,000 marks, give the crusaders food and money for their journey, join the crusade himself with a Byzantine army of 10,000 men, and support the Holy Land with a garrison of 500 knights. All of this was for the small price of attacking Constantinople to put him on the throne. In 1203, they achieved this in a roundabout way. Alexius was the emperor, Alexius IV, but he was co-emperor with his father Isaac. Isaac agreed to keep his son's promises, but this was ultimately impossible.

In the 20 years before the 4th Crusade, the Byzantine Empire had gone into a steep decline. It had become politically, socially, and economically fragmented. Rebellions cut off swathes of territory from the capital, and the emperor had been largely unable to raise taxes beyond the area around Constantinople since 1201. The empire was broke. Alexius IV tried to raise money by melting down treasures in the capital, but this only raised half the money. It also provoked rioting, and Alexius IV was overthrown and strangled by Alexius V.

This left the crusaders without most of the money and resources they needed. They were also beginning to go hungry, since their supplies of food had been cut off by the new emperor. Earlier in the year, the leaders of the crusade met and decided to take the very bold step of breaking apart the Byzantine Empire, and agreed to split it between them in the event that the emperor failed to keep his promises. On 12 April 1204, they enacted this plan and attacked, and took the city in about 24 hours.

So the attack on Constantinople was not deliberate, it was the product of a combination of Byzantine politics and crusaders who did not understand the logistics and finance of military campaigning. There is a lot more that could be said about the diversion of the 4th Crusade, but what I've described is the basic chain of events and decisions that led to it.