Because it is like that. Sakurai has released statements about how he's avoiding cutting veterans if he can, as well as that there won't be as many newcomers this time. The roster is finalized and he wants to have a finite amount of characters, considering that he already found Brawl's roster size to be perfect the way it was.
And what, sales is what determines the roster now? Are you kidding me? If sales really is the defining factor here, why does Earthbound have more than one character, or Starfox have three for that matter? Sakurai looks at series and chooses fitting characters for the overall cast balance. He's adding characters, not entire franchises summed into one moveset. He's representing series, not sales figures. Mario is a big part of Nintendo, but it's a false statement to say Mario IS Nintendo. Such logic dictates that Mario offers the same kind of experiences you could find in a Zelda game, or a Metroid game, or a Starfox, Mother, Pikmin, Pokemon--a huge number of series, and you'd be ignorant to say it's not as such. Nintendo would lose a chunk of what it has without Mario, but it'd still be Nintendo, spirit and all.
One man is selecting the roster, and it's not Miyamoto himself or big corporate businessmen. It's Sakurai, the same guy who decides adding obscurities like Mr. Game & Watch and R.O.B. to the roster will give a more rewarding end-product than a more well-known yet generic addition. For the most part, he's right. He knows that adding lesser-known series is a huge boost to their popularity as well. Why else would Fire Emblem have the popularity it has in the West now? Why else would Earthbound have a cult fanbase that turned into a full-fledged giant community? Mario doesn't need the boost to gain popularity, these other games do... That's why insisting we need even more Mario characters simply because we already know them is a selfish request, both to other franchises and their fanbases.
Edit: I ended up putting so much soul into this post that I sold it to the devil. Post #666. **** is real, now.