Why was the first crusade successful in the fourth crusade not successful?

The First Crusade was successful primarily due to its strong motivation, unity among the crusaders, and the element of surprise, allowing them to capture Jerusalem in 1099. In contrast, the Fourth Crusade (1202-1204) deviated from its original goal of reclaiming the Holy Land and became mired in political intrigue and conflict, notably leading to the sack of Constantinople instead. This lack of a clear, common goal and the internal divisions weakened their efforts, ultimately resulting in failure to achieve their intended objectives.