Its more like the standards for moving down are pretty low. Most people talk about the potential of characters to drop more than of rising. Just the overall mentality of "Anyone else besides top tiers are bad".
And part of it is true. A lot of characters have nowhere to go but down, with the people who rise up usually because you need someone to occupy the now empty spot.
Condensed Version:
S: Meta Knight
A: Snake (Nowhere but down), Diddy Kong, Falco, Ice Climbers (nowhere but down), Marth, Wario
B: King Dedede (nowhere but down), Pikachu (nowhere but down), Olimar (nowhere but down, Lucario (ditto), Mr. Game & Watch (ditto)
C: Pit (ditto), Toon Link , Zero Suit Samus, Kirby (nowhere but down), R.O.B.
D: Donkey Kong (nowhere but down), Peach, Fox, Luigi, Wolf
E: Sheik, Pokémon Trainer, Sonic, Ness, Bowser
F: Lucas, Ike, Yoshi, Mario, Falcon, Samus, Jigglypuff
G: Zelda, Link, Ganondorf
There is little potential for most characters out there to go up anything more than a spot that isn't related to being bumped because of other cast members being placed below you. The members in the A tier will always be there, and will probably just shuffle a few minor spots with each other. Like Snake maybe dropping down, Wario going back up, Marth over IC's, etc. IC's have the most downside obv.
Then below that, B tier is pretty much set in stone unless you expand what qualifies as B tier. Dedede certainly will never rise to A tier, and there is good reason to think Olimar and Pika might be better than him at least matchup wise. On the same token, Pikachu and Olimar will also never make it into A tier, so the only place they could move is down. Most likely they will be the top of that tier, with either Dedede/Lucario behind them, and G&W at the same spot.
G&W definitely has nowhere to go but down. The only character I could see ahead of him though would be ZSS. I don't think Pit or TL have enough in them to really go above the simple but powerful stuff G&W has. Pit probably isn't going up, same with TL. Yes they have potential, but they aren't as good as those above them. Then you get into Kirby and ROB, two characters almost certainly worse than those ahead of them. No gain expected for them either.
Then you have DK. Where should he go? Realistically probably lower because of Dedede. Even without Dedede in the picture, his viability is shaky at best in Brawl. His matchup spread certainly isn't anything amazing. He has better tools than most of the people below him, but he also has much bigger weaknesses. I think the negatives outweigh the positives for DK, no chance he's going up. Peach is in a similar boat; she has potential but do you really think she is better than, say TL or Kirby? I could see her going above DK and ROB at best, with that kind of move being a stretch. Past that point, you delve into champs with multiple ridiculous matchups that frankly shouldn't be viable competitively, but manage to squeak in every so often. The Fox's and Luigi's and what not. Everyone at this point is kinda like A tier; minor shuffling of positions here and there. No one would really move up, just people moving around depending on who you think is worse in each case.
Then you get into low tier.
At this point in Brawl's lifetime, I honestly think MOST tier spots are played out fairly accurately; I do not think there is a single character on this tier list that is a full tier too high or too low (meaning ZSS/TL for example might go from mid/top of C tier to Bot of D tier, but not to the middle of D tier. Or Pit might drop but not all the way to the end of his tier/into the next one). I think that is pretty good to have achieved that. Most shifts that would occur would be opinions on metagame shifts where our analysis of a character's traits isn't changed, but certain things might be seen as more valuable than before, or not as important as before making a few shifts occur. I don't see any cases of a character drastically rising or falling coming up anytime soon.