Platformers are declining in general and Galaxy isn't as pick up and play as NSMBW, or MKW. People like myself play in short bursts not long time periods.
Also, frankly I just didn't find Galaxy fun. Neither did my son and he loved Mario 64 and played through Sunshine. It was well done, and entertaining enough while playing it but it wasn't the sort of addictive fun NSMBW, Mario Kart or even Mario Strikers was for me.