Well, I'll play devil's advocate here and say I love that it works this way because the Tournament Coordinator in me is thinking that an Achievement Challenge next year is a no brainer and will be fun. Challenges have to be as fair as possible for everyone and if achievements were not wiped clean then you'd have some players who have been playing the game since day 1 and have racked up lots of achievements already competing against other players who have just purchased the game.
But, I get what you're saying, Fysh. It's going to be tough to get a ton of achievements in a single game unless you're looking at a longer Dynasty/Legacy kind of game.