I know that the reasons for starting the crusades were mainly political, but were the people who did the fighting motivated by religion? What about the people's crusade, wasn't that not tied to any of the nobility? Would there be practically no political motivation for them? Also, if one of the major political reasons was to expand the Byzantine empire against the Ottoman, why did western Europe want to see a strong Byzantine Empire over a strong Ottoman Empire? If there was no religious motivations, than why do we associate them with christianity so much?
So I've said this before:
The negotiation that goes on between religious belief and Realpolitik in relation to the Catholic Church is always an interesting one to study. Part of it has to do with the fact that the modern idea of "religion" does not really apply to the pre-modern period. The Latin religio means something closer to "ritual practice" or "supernatural obligation," rather than a concrete noun. And when you find it in the medieval context, it is almost always being used to mean "a person in a monastic vocation" (as opposed to the secular---non-monastic---clergy). To drive the point home: a "secular" person in medieval Europe meant a priest, deacon, canon, or bishop. The evolution of the modern meaning of "religion" (i.e. as one of many orthodoxies, rather than a singular orthopraxy) is the product of post-Reformations religious pluralism and the creation of a European global awareness. Gil Anidjar put it best: "Secularism is the name Christianity gave itself when it invented religion, when it named its other or others as religions." (1)
All this is to say that "religion" in the modern sense was something that was taken for granted in the pre-modern period in a way that it is not any longer. Many things that appear to us as ironies can be explained by this disconnect.
For the specific question:
The motivations of the crusaders is manifold and complex. We have no contemporary, first hand account of Urban II's actual sermon that launched the First Crusade. Those we do have are either written by eyewitnesses after the Crusade's success or by those who had heard about it. Both types are biased by the outcome of the Crusade and their own agendas. Moreover, even if we could pin down exactly why Urban launched the Crusade, it would not tell us why so many went on it, nor would it help to explain why crusading in general continued.
With this in mind, I think the best answer is that we shouldn't try to pin down the "main" reason why there were crusades and crusaders. The people who went on crusade were certainly motivated by religion, and "religious" motivations were not distinct for them as they are for us. This does not mean that they did not have other motivations, nor that those other motivations would exclude religious motivations. We see "political" and "religious" as concepts somewhat opposed to each other, and this is the product of a modern idea of what "religious" means.
(1) Gil Anidjar, "Secularism," in Critical Inquiry 33 (2006), 62.