Majin-Tenshinhan on 23 April 2009
As most people here said, they've already got all the important Mario characters. Same reason no new Zelda characters came. Mario and Zelda are two of their biggest franchises, but there's really nobody important left to add from them. The "big 4" in Mario are Mario, Luigi, Bowser and Peach. They're the center of most games. Then you've got the very popular Yoshi and Wario, who've even branched out into their own franchises.
With Zelda, it always centers around the Triforce, thus Link, Zelda and Ganondorf. That's why the fourth Zelda character is just another incarnation of Link. There's nobody else "worthy" to be in.
