Good point. It would be pretty unbelievable to have Peter Pettigrew become god-like over night, because he doesn't seem to be the type that's motivated enough to earn that power. Voldemort, we can safely say, would definitely work towards having that kind of power (and did, in a way), so it's much more believable for him to become the arbiter of doom for the HP world.

Really, I just like to see characters (whether hero or villain) having to work hard to achieve the power they need/desire.