Huh? What does achievements have to do with resetting the economy?
You really aren't the sharpest tool in the box, are you?
EDIT: Let me rewrite this post since it seems many don't understand the overall concept of previous Diablo and the new one.
The reason Diablo II had difficult leveling and a ladder was because of lack of longevity. Once you hit hell mode all you could do was Baal run for meaningless improvement of the character.
The man failure were the runewords that were not supposed to exist in such a large measure. The inflow and outflow of items through the environment of third party sites, duppers, botters and so on, inflated the economy to ridiculous lengths. This was the reason why Blizzard chose to reset the ladders. Because a non-reset ladder would lead to stagnation of items and trading, if you will. You have an Enigma, I have and Engima, that guy has an Enigma. Everyone has Enigmas.
Reset gave the chance for people to work and reacquire the items.
The ladder, as mentioned earlier was used for e-peen purposes. Trust me, I know. It was the thrill of the race, and many never made it to 99.
But a ladder in Diablo III would be utterly meaningless. The max level will be fairly easy to be achieved. What would lead this to? Having 10.000 level 60s in the top of the ladder? The mechanics simply don't work. Further more, whether we'd want to or not, it would affect the players and divide them.
All of this has been now traded for achievements. So that players can acquire gear as well as making stupid achievements to show off.