They didn't want the cameos to steal the spotlight. The staff said with the classic characters, they've been done so much over the decades that they all have their own personality already pretty much and that would overshadow the main characters (Ralph, Felix, Vanellope, Turbo, hi-def FPS lady). It's different from Toy Story. Mr. Potato Head never had a personality, so they could make one up for him. Barbie and Ken have always been portrayed as having plasticy personalities by Matel themselves, so it was easy to incorporate them into TS3. Army Men were, well, army men. But with game characters, their personalities are too firmly routed in gamer culture. That's why Bowser and Eggman never talk, Kano's pretty bland, and Clyde -- who never talked outside the cartoon series -- was given the lines.
Anyone else notice at the end of Wreck-It Ralph, Zombie looks like he's jonesing or maybe even just shot heroin before the meeting?
And I kinda like that Mario didn't show up, just had a name drop. "Fashionably late as always." That was such a great, subtle joke. Took me forever to finally get the punchline, but once I got it it made me smile. I can't reveal it, because it'd ruin the joke.
This always makes me smile every time I watch the movie:
Wreck-it Ralph : Adorable winner