Super Mario Galaxy is the highest reviewed game on Wii by the critics (it has a 97 on Metacritic), but it certainly hasn't sold very well in comparison to the Wii's best selling games Mario Kart Wii, Wii Sports, and the other Wii Series games that have Metacritic scores ranging from 15 (Mario Kart Wii) to 39 points lower (Wii Play).

    Also, New Super Mario Bros. on DS has almost sold 20  million copies proving that Mario platformers still do sell extremely well.

    So, why has Mario Galaxy only sold 8.08 copies so far? 

"only sold 8.08"
You make it sound like thats nothing.

Sales doesn't equal quality.

     I think it's small in comparison to these numbers:

Wii Sports -- 47.13 Million Sold -- Metacritic Score 76 ( 100 being best)

Wii Play -- 24.03 Million Sold -- Metacritic Score 58

Wii Fit -- 21.60 Million Sold -- Metacritic Score 80

Mario Kart Wii 17.13 Million Sold -- Metacritic Score 82

Super Mario Galaxy 8.08 Miilion Sold -- Metacritic Score 97.

lol, you make it sound like the crappier the game, the more it sells (cept' wii sports)

It has sold well, and has more to go with the legs it has.

As was pointed out- The mario title + incredibly high metacritic score failed to sell more than titles that are minigames and fitness trends.

Most of the people who buy these games don't even know what metacritic is.

its a Single Player game( and im not counting 2P being pointer guy)

It's a good question, and I wish I knew the answer. It does seem that it will never catch up to Super Mario 64. Although, Super Mario 64 did have the benefit of being a launch title.