Here's why:
* The Crusades occurred between the 11th and 13th centuries. Robin Hood, as a figure in folklore, is generally placed in the 12th or 13th centuries, but his actual existence is debated. Even if he did exist, his timeline would be too late to overlap with the Crusades.
* Robin Hood's legend centers on his actions in England. He's known for stealing from the rich and giving to the poor, specifically within the context of Sherwood Forest and the oppressive rule of Prince John. The Crusades were fought in the Holy Land.
While some fictional stories may depict Robin Hood taking part in crusades, this is not part of the traditional legend and there's no historical basis for it.