 The Prophets, being immaterial creatures who do not experience time linearly, found Zek's obsession with accumulation of material wealth to be strange and adversarial.They "de-evolved" him to a more primitive state resembling the less greedy ancestors of the Ferengi.They threaten to de-evolve Quark as well, but Quark warns them that if he is also changed, more curious Ferengi will visit the wormhole looking for answers.