I could be wrong but I suspect that achievements are highly appealing to a small portion of the (already small) vocal minority that call themselves "hard-core gamers" ...
When I was a teenager there was a handful of games I devoted significant effort towards completing all of the objectives and getting all of the unlockables; but this was a rarity that was (typically) reserved for the best games that had particularly meaningful unlockables. At the time it was common to read reviews about games that included comments like "There is lots of replay value; if you complete the game with only a pocket knife you will get the beret for your character to wear on the next play through"; and I see achievements as being less meaningful than a superficial styling change to a character.
Now, I'm not saying that achievements are a bad thing (I don't have a problem with them being included in a game or a system) but I suspect that Nintendo's reluctance to create a system like this is that there is a significant cost and they don't believe that there is a significant benefit.
