Because compared to Super Mario Sunshine (the Gamecube Mario Flaghsip title) it looks amazing and innovative.

Its a mario game. dont expect much of a story from it. It is also so much fun.

Don't you dare knock off Sunshine.

When I play Mario like I have for 23 years I don't want to read a novel. I enjoy the great control, and galaxy has great control and the gameplay.

And why are you confused? I don't think you are confused you just don't like the game.

And the graphics are great HD or not

People being preoccupied on graphics is why gameplay is taking a backseat. The games while frustratingly difficult at times, were much better during the 8-bit and 16-bit eras.

That's just your opinion, I enjoy 3D gaming a hell of a lot more then 2D gaming.

I'm not saying I don't like 3-D gaming. I'm just saying the gameplay was better, and more challenging before than it is currently overall.

you seem to be missing the point

I fail to see how gameplay was better, you have a lot more gameplay options with 3D gaming. Of course 2D gaming can be as much fun as 3D gaming(super smash bros, muramasa,etc)
I just like 3D gaming a lot more overall.