Smash Bros problem is it's impossible to ever do right online. Most 2 player fighting games last gen had problems working right, and these were games with plan, non interactive stages, and no items. And now you have Smash Bros with 4 players, items, stages with lots of changes and interactive things going on.
This is why Sakurai originally said Brawl would only be 2 player online because 4 player would be too hard. Of course he was able to get 4 players working, but as you all see there's a lot of lag, which is what was bound to happen for a game like Smash Bros if it was done with 4 players, items and all stages online.
Mario Kart on the other hand isn't anywhere near as hectic as Smash Bros. Not to mention Nintendo has design Mario Kart Wii around online in the first place, something Brawl wasn't. Why else do you think Mario Kart Wii has a much much better online structure then Brawl did.
I mean hell, Nintendo is giving Mario Kart Wii it's own channel on the Wii. Just read about it in the recent Ask Iwata interview on Mario Kart.
http://us.wii.com/iwata_asks/mariokart/vol1_page5.jspThis should show you just how comitted they are with Mario Kart Wii's online play.