Honestly, Sonic has many, many aspects that scream that he deserves EXACTLY this placement on the tier list. And few, if any, that state otherwise.
I mean, he's supposedly a "fast" character, but anything he does (smashes, aerials, aerial movement, tilts even) comes out relatively slow. But he still hits like a fast character, having very little kill potential. The problem is, he doesn't hit like a GOOD fast character, who has a number of moves that keep the opponent close enough to follow up. Basically all of his moves, across the board, have exactly enough knockback to not be combo-able, but at the same time have pathetic KO and damage potentials. Add this to the fact that they don't come out that quickly and you have a character with a blatantly bad moveset.
His recovery looks like it could make up for it, to an extent. But keep in mind that Sonic is one of the few characters incapable of up-Bing straight to the ledge. This makes him extra-vulnerable to things like spikes and edge-guards.
So we've got a character with bad KO potential, bad combo potential, bad straight damage potential, no projectile or answer to projectile spam, and a fairly mediocre recovery. And his one "uber" quality (speed) only applies if you're running all the way across the stage, due to his slow aerial movement and not-great acceleration. So what exactly makes him deserving of being higher on the tier list? The fact that you can play him well? This is brawl: any character can be played well, and can even place in tournaments if played well enough. The tier list is more a measure of the character's overall power-level, which in sonic's case, is inarguably fairly low.
