Fox's top tier in melee is due to the power of having a truly infinite combo available (Drillshine) and a powerful, quick midair finisher (upair), one of the best smashes in the game (Upsmash), a good projectile as well as two sex kicks.
If you look at the top two tiers of Melee, they basically all have some sort of "cheap" damage building technique (Pillaring for falco, chainthrowing for Sheik, Marth, and Peach), a powerful, quick midair finisher (falco's spike, sheik's slap, marth's spike, peach's slap), a powerful and quick smash (forward for falco and marth, down for sheik and peach), a good projectile (except marth, but his sword is long enough range already), and two sex-kick style aerials (neutral and back for sheik, peach, falco. Marth doesn't quite have sex kicks, but his fair and bair are pretty beastly and quick, so they make up for it).
Now if you look at Brawl Fox, he has No Drillshine, a weaker Upair, a weaker upsmash, about the same projectile (minus the short-hopping part that made it so good), and weaker sex kicks. Sure, he's still got speed and power enough to end up high or middle tier, but without the degeneracy of the Drillshine...