Hi there! I wrote about the political situation in Europe at the time of the Fourth Crusade in this post, although there's always more to discuss.

In short, in 1200 Western Europe was rudderless. The King of Sicily was a five year old boy in the care of Pope Innocent III. In Germany, Otto of Brunswick was in open war against Philip of Swabia for the title of Kaiser. France, although ruled by the energetic Philip Augustus, was split between the Capetians and the Angevins, who were themselves coping with the death of Richard Lionheart and his successor John, whose epithet "Lackland" didn't exactly promise effective rule. In the east, Emperor Isaac II, after ten years of internal strife, had been dethroned by his brother, who had gained the support of the nobility through favors which in addition to his own incompetence made for extremely ineffective rule. Not exactly a political environment that would rush to Constantinople's aid; and besides, there was little sympathy in the west for the Empire to begin with. The Byzantines had even been accused multiple times of betraying the cause of the crusaders over the past century, and had long since stopped being an important player in Europe. Even Pope Innocent, the only one who had publicly complained about the redirection of the crusade changed his mind once he figured he would be able to unite the Eastern and Western Churches thanks to the reorganization of the Empire.