1. Oh. My. God. At what point is Slime "sorta popular" in Japan?
Besides the games themselves, he's the most heavily promoted product related to the DQ series. You are clearly 100% unobjective, and refuse to even acknowledge that the mascot of a franchise that looks down on KH from its throne. KH, at no point in its history, could even suggest that it could ever catch a glimpse of DQ's success. DQ's spin-off, DQ Monsters, is bigger in Japan than KH is (DQ Monsters Joker for the DS sold 1.7 million copies over the span of four months, compared to Kingdom Hearts II's 1 million). See, it's not actually me discrediting Sora's popularity anywhere, it's you trying to brush Japan to the side whenever you discuss popularity.
You don't care for Slime much at all, at no point can you give an objective perspective on the success. But I can - and just because I point out the fact that Slime is a cultural icon in gaming in Japan doesn't mean it's my opinion.
2. And you seem to ignore that DQVIII sold 1.2 million copies outside of Japan, meaning that it went from DQVII being a bomb to DQVIII being a million selling game. Do you realize that DQVIII is the first DQ game ever released in Europe, and it managed to sell 600,000 there? Considering KH has FF and Disney appeal, the fact that it outsold it in those regions doesn't even matter. We've already got a character that is not popular to Japanese audiences (Sonic), and a character with popularity in both.
3. And yet, I must reiterate that no matter the popularity, Sora's popularity exists OUTSIDE of Nintendo. So why would Nintendo go through all the licensing issues with Disney to get this character, and in all likelihood, several other characters? SSBB will not have any Disney-oriented characters in it (aka, characters from Disney cartoons), because of the rule of having to be a Disney character to get in. Which means that if Sora was included, nothing of his moveset could hint at anything Disney. Almost every character in the game would be inapplicable - the whole attraction in KH is the Disney element, so what would be the point if he would lack the Disney element?
4. Um, okay? That hasn't been established, ever. The fact that no pcs in Brawl establish that this character can get in is because there is no better choice that is the mascot of the series. They are either the mascot or don't have one. Slime is the most recognizable anything in DQ. That is fact. Like, we can establish this easier than we can establish that the theory of gravity is true.