Speculation on Marth's Appearance in Super Smash Bros. Melee/Brawl
Note: This page is speculation and should be treated so.
Marth made his second appearance in the Smash Bros. series with Super Smash Bros. Brawl. Although it's only speculation, Marth was kept in the game unlike Roy because he is the main character of the game to start the Fire Emblem series, and even moreso because he features in the FE1 remake Fire Emblem: Shadow Dragon for the DS.
His appearance in Brawl can be further backed up by the fact that he is the main character in the DS remake of the original Fire Emblem, and his inclusion in a game like Brawl would increase awareness of the game. Simply put, he was used for advertisement. Psychologically people may use Marth during Brawl and become curious about his origins. Fire Emblem 11, for the DS, would answer that question, and players would be interested, thus expanding the success of FE11's release, the ultimate goal for any company.
To find out more information about Marth in Super Smash Bros., please come here.