It's the attack move that breaks combos and multi-hit attacks extremely reliably. Since you can attack out of hitstun so early in Brawl, Nair is the move, since it starts up so instantly and carries it's good priority, that breaks you out of utilt combos at like 16% where it goes up to 32% on other characters. Breaks you out and -damages them- to boot.
Here's an example of Nair's use- Wolf's up-B move, between the sweeping hit and the last kick, if you spam neutral air you'll fit a sex kick in their and hit Wolf out of his move between the two hits. You can reliably fall back on this move almost all of the time and have it work.
Also, it goes through projectile hitboxes, keeps going, and then hits the opponent, as in the case of Olimar's/Snake's Upsmash, or Bowser's flame breath even. I also think a nair might also be Luigi's fastest out of shield option outside of a shieldgrab.
Lastly, strong hit Nair does 14% damage, sweetspotted it can kill, weak hit nairs have followups because of the move's little landlag.
It is not that Luigi is the only character who retained his sex kick from melee, it's also that Luigi's nair was the second best sex kick in melee too.