But that's just it. A square is a rectangle but a rectangle isn't necessarily a square. There were only two Zelda stages, but four Zelda characters. That left two without stages of their own. Conversely, there were two/three Yoshi stages, but obviously only one franchise representative. The same situation presents itself with DK. Also note that, while Balloon Fighter appeared in the Melee beta, his alternate music on Icicle Mountain would have been the only representation he had as far as stages go.
To include a stage, the team includes music to fit and a character to utilize said stage. Other characters that don't get stages are still represented with alternate music. Granted, this is based on Melee's system, but I think the general idea will hold true for Brawl.
The inclusion of a stage always means the inclusion of a relative character. The inclusion of a character does NOT always mean the inclusion of a relative stage. You're driving the opposite direction down a one-way street.