Why does Chaucer dwell more on the Pardoner's love of money than on any other aspect of his character?
The most likely reason that Geoffrey Chaucer would have done this is because of the reputation that pardoners had in the times when he was writing.
The job of a pardoner was, as the name suggests, to go around selling pardons, or indulgences as well as relics of saints. A person in those days could buy an indulgence and be guaranteed forgiveness of sins in return. This practice was one of the things that would eventually lead Martin Luther to break with the Catholic Church.
Pardoners had a bad reputation because they preyed on the fears of their customers. They used their customers fear of damnation to make money. Therefore, they had a reputation for being greedy and unscrupulous.