simple... more voice acting would be annoying. The very little that Mario does say in Galaxy is more than enough to be somewhat embarrassing. But, no.. why develop a story for a game that is meant to be a skill testing, fun platforming romp?? There are plenty of other games that do need involved stories (take Eternal Darkness or the Metroid Prime series for example). Must every game produced have huge, complex story lines, lots of voice acting, and meet a minimum number of requirement of gameplay mechanics? Where does that end? Let games stand on their own and accept them for what they are meant to be. If you want some other aspect to your gaming, try a game that is built around that... (only makes sense). There are plenty of choices out there! (Thankfully)
Example: I think Marble Mania on the Wii is a very well done game. Guess what.. No plot/story AT ALL, nor Voice Acting (well, except maybe the noises from the little dude in the gyroball) And, putting a story or voice to that game would have just be flat out stupid and a waste of time.