sometimes it's the search for more spin. other times it's the search for the right bounce/grind configuration. recently it was just because mine were a couple years old and i was really interested in the cobra pur wedges, but when they didn't work out i took a chance on an unknown and it's been working out well for me.
Lack of spin. I generally try to go 3 years between wedge sets, though it should only be two (and maybe even less than 2 years between wedge sets). There was a video or thread somewhere about spin loss after X number of rounds with a wedge, and it was pretty enlightening.