Because the old ones are just as exciting, as shown by their superior popularity overall (Marth and Sigurd are more popular than any FE character save Ike alone in Japan). Being new doesn't make them better as characters or as representatives for SSB. Indeed, the fact that having just those three would mean that only a very small portion of a diverse franchise would be represented is a very good reason why that would be a bad set of choices for the series.
Fire Emblem isn't like any other Nintendo franchise. Its stories and characters shift with almost every game, and thus it can't be appropriately represented just by characters from one set of its games. Mario, Zelda, Donkey Kong, etc can use guys like DK, Link, and Mario and be representing the entire series perfectly with just a character or two because those characters are constants throughout the series. Fire Emblem cannot do that, and thus its important to make sure that its representatives come from a variety of its games, not all the same ones.
And why would you think that? Marth was the primary FE rep in Melee, was he not? And theres nothing stopping from being in Brawl, is there? Clearly, Sakurai is listening to the fans - we have evidence aplenty of that in Sonic, Diddy, Pit, Metaknight, and Ike. He most certainly does care.
Zevox