Why did so many people leave on the first crusade?

Answer 1

There seem to be a variety of reason - Jonathan Phillips seems to sum this up well when he describes there as probably being as many reasons for going on crusade as there were crusaders. Though the charters seem to overwhelmingly suggest that religious piety was the predominant factor in willingness to go on Crusade. Whilst the papacy offered to ensure the safety of property and possessions as part of taking up the cross the actual legal affairs regarding this weren't sorted until at least the end of the second crusade and there are examples of bishops actually referring crusading court cases to secular officials because they do not know to what extent their jurisdiction reaches.

If you'd like more information on this then I highly recommend Jonathan Riley-Smith as a source on the justifications for going on crusade. His books are well renowned so you should not find it difficult to come across them. In particular his book

The First Crusaders 1095-1131

Pages 112-163 should be of help. (The 1997 version)

Hope that's sufficient!