This was partly because Urban wanted to unite Europe, and he'd figured out the lesson the rest of us learned from alien invasion movies. The best way to get people to unite is to give them a common enemy. So Urban called on all the bickering knights and nobility of Europe, and he saideth unto his people, Let us go forth and help the Byzantines, because then maybe they will acknowledge my awesomeness and get rid of their stupid not-having-me-as-Pope thing, and while we're at it, let's liberate Jerusalem. I'm paraphrasing, by the way. Shifting the focus to Jerusalem was really important because the Crusades were not primarily military operations. They were pilgrimages. Theologically, Christianity didn't have an idea of a holy war.