Why was the pardoner able to become so corrupt?

The Pardoner is an excellent orator and gifted manipulator. After he explains where he's from, he displays his papal bulls, indulgences, and glass cases crammed full of rags and bones. He then tells the congregation, they are holy relics with magical properties.

Then, the Pardoner invites anyone who has sinned to come and offer money to his relics, and therefore to be absolved by the Pardoner’s power. This trick, the Pardoner says, has earned him at least a hundred marks since he was made a pardoner - and when the “lewd peple” are seated, he continues to tell them false trickeries and lies. His intention, he says, is simply “for to wynne” (to profit), and “nothyng for correccioun of synne” (and nothing to do with the correction of sin); the Pardoner doesn’t care whether, after burial, his congregation’s souls go blackberry picking. Thus, the Pardoner says, he spits out his venom under the pretense of holiness, seeming holy, pious, and “trewe”. “Greed is the root of all evils”, the Pardoner quotes again, explaining that he preaches against the same vice which he himself is guilty of. Yet, although he knows he is guilty of the sin, he can still make other people turn away from it.