Information comes from Nintendo’s Bill Trinen…

“We’re not opposed to Achievements. When they create their games, [Nintendo’s designers] don’t tell you how to play their game in order to achieve some kind of mythical reward. Basically, the way the games are designed is they’re designed for you to explore the game yourself and have this sense of discovery. To that end, I think that when you look specifically at games from EAD [the group long led by Mario and Donkey Kong creator Shigeru Miyamoto] and a lot of other games that Nintendo has developed a well, there are things you can do in the game that will result in some sort of reward or unexpected surprise. In my mind, that really encourages the sense of exploration rather than the sense of ‘If I do that, I’m going to get some sort of artificial point or score that’s going to make me feel better that I got this.’ And that, to me, is I think more compelling.”

Weirdly I am ok with this reason. I guess if you look at it that way Brawl had a lot of achievements.

Link