There are two likely explanations.
1. Nintendo would have put more into it if they knew how high the demand would be.
2. Malstrom is right, and Nintendo doesn't see 2D Mario as worth the effort (because they like making 3D Mario more), which is why Galaxy 2 got all the polish over this game and NSMBWii.
The real answer is probably this: Some random guy at Nintendo thought "hey we could do something for Mario's 25th anniversary!" and they had to make a package that was worth buying in less than 3 months. So all they could do was pack the SNES game on a disc and put effort into the booklet and package. Fear of bad Christmas sales probably helped here.
This game definitely got priority from a production standpoint: The box looks really great, they made a cool manual for it, the booklet is high quality and the soundtrack CD looks cool. (And it doesn't look like the people responsible for music wanted to show their "genius" with the CD, really. It is just a random collection of songs and the Mario Galaxy soundtrack is definitely one of the most amazing soundtracks in any Mario game)
All that "booh Nintendo hates 2D Mario" is just a stupid Malstrom thing. Last year we got a 2D Mario, this year we got the classic collection, a Mario bundle (and a Donkey Kong Country) and I'd take any bet in 2012 we will see a new 2D Mario for the DS. 2D Mario is probably the most used AAA franchise in Nintendo's developer kitchen right now.
Edit: That also reflects my opinion regarding the OP. I would'Ve liked some more sugar and spice in here as well but I guess the game didn't have much, if any, development time. And I'm really happy we got it I love Allstars