Because of the Disney hurdle. People are putting a lot of stock into this causing Sora to have little or no chance. Also people using the example of KH3 not being on the Switch. Yet we forget Cloud was in a similar postion, he was seen as impossible. His game was not on a Nintendo console at the time, nor were the two newest Final Fantasy game coming to a Nintendo console. It was such a surprise when he was released for Smash Wii U that people were genuinely shocked and excited.
Sora has the potential to be the same. His newest game is not on the Switch and people believe Disney is too big of a hurdle . If he made it in, I do believe he would receive the same amount of shock due to some hurdles Nintendo would have to go through to get him.
OK, but if you don't know about the Disney license, then its not hard to believe. Even with what you mentioned, Sora gets brought up all the time both here and in other places (there was a big thread about it on 4Chan last night). His inclusion wouldn't be a shock.
The problem is that after Cloud, you aren't going to get a lot of surprises because now people expect every major third party character. The only way it could be is if it was someone that was never on the radar. From Square Enix, maybe you could say Octopath Traveler would be unexpected but thats because no one brings it up (though I expect if there is a guest character, it would be from that game).
The most surprising characters this game were from Nintendo. No one expected Ridley and K Rool because they were snuffed in the past and Piranha Plant because no one would have guest it. Simon, on the other hand, was pretty much expected and would have been even without Verge's leak (after Snake and Bomberman, who are you going to pick from Konami). Richter was more surprising as he was an echo of Simon and people didn't think he'd get one. If you are going off unexpected, I would pretty much any guest character is expected at this point and Sora definitely is.