It had to be limited to Melee and Brawl because we'd like to use the most compelling and exciting video to promote Smash (and get more people to play it competitively). Clips of the original aren't going to really inspire most gamers out there. It looks visually unappealing to anyone unfamiliar with the game, and might give the impression that hardcore smash is all about a game that few people outside the community play anymore. While Melee itself is fairly old, it's visually very similar to Brawl - it's more plausible as a game with a strong community.
It's harder to draw people in with the promise of playing such an obviously dated game that has a small competitive community. For example, there are people that still play Street Fighter 2 seriously, but that doesn't draw interest like Street Fighter 4 does these days. That's just how these things work.
As far as Brawl+ goes, I will try to have an answer by Wednesday. I wouldn't rule it out quite yet.
It's harder to draw people in with the promise of playing such an obviously dated game that has a small competitive community. For example, there are people that still play Street Fighter 2 seriously, but that doesn't draw interest like Street Fighter 4 does these days. That's just how these things work.
As far as Brawl+ goes, I will try to have an answer by Wednesday. I wouldn't rule it out quite yet.