I think the thing is, casual gamers are different than us. "Us" as in people who are longtime gamers, the Internet regulars, the kind of people who know details within half a day of them being released. There's a clear distinction when you consider that the Xbox 360 is considered one of the "hot gifts" this Christmas season, but it seems like every poll on GameFAQs regarding "do you have an Xbox/Xbox 360?" or "Are you considering getting an Xbox 360?" always seem to end in 50% saying they aren't interested in it at all. The GameCube may be behind the Xbox in sales but looking at polls like that on general gaming web sites you wouldn't think so. On certain questions the GameCube can be really close to the PS2, even. Web-savvy, hardcore gamers =/= general populace, unfortunately.
Anyway, I think my point was, every bit more info that's released eventually drips down to some of the general populace, but it truly doesn't hit until the marketing begins. I'd bet there are quite a few people who are looking forward to the PS3 who aren't even aware a MGS4 trailer even exists, for instance.
But for those at-least-slightly-web-savvy gamers who are at least aware of gaming news info on the web, it is easier to point to a MGS4 trailer and get an idea of what is there. Even if you've never played previous MG games you know what it is. If someone had never played a SSB game, they couldn't have much excitement about the next one before any visual confirmation of the game exists. Plus, a screenshot or trailer is more memorable than a tiny snippet of info to someone who isn't a major fan. If Nintendo's going to try and create more fans, just a bit of info isn't enough. Confirmation that a new SSB is in the works really only keeps the SSB fans intrigued, you need at least a trailer to get non-fans to even notice it.