Because there are plenty of other choices for distinct characters, and the roster slots for unique characters, this time ARE very limited given the tight dev time. All in all, it makes more sense for Sakurai to go for a ballot pick and please a larger number of people, than take a risk with a new Pokemon just for the sake of tradition.
Not saying it can't happen. But I don't think a new Pokemon is necessarily a given fact, considering how many Pokemon already make up the roster. They're lower priority than any top 20 ballot character imo.
Tbh, I never paid much attention to the story in SF. So that's fair I suppose. I still think Ken is a better pick/more likely choice than Akuma though.
Just.... saiyan.
Most of the new Pokémon are from Gen 1. Only two are from later games.
However, your claim is pretty subjective. Yes, Sakurai can get all the newcomers from the ballot, however if he finds that a character from a more recent game (I.e Pokémon Sun and Moon, and Xenoblade X) to be more distinct and simply likes them a lot more than a some of the ballot characters, he will do just that.
Of course the ballot will be taken into some consideration, but as well as some of the recent characters, and Sakurai already pleased a lot of people with some of the characters like Inklings, King K. Rool, and Ridley. There are probably others people want like Isaac and Geno, but if Sakurai simply doesn’t find them interesting enough to work with (which IMO he should add Isaac) he simply wouldn’t and look for another character.
Sakurai doesn’t just add characters people really want at this point, he looks at all of the stuff available and picks characters who he thinks people will have fun with, and this includes games like Pokémon Sun and Moon which is a franchise a lot of people like, and who’s characters have become favorites.
There ARE a lot of people who want Gen 7 Pokémon, some for the sake of having more pokemon not from Gen 1. It’s just a goldmine for adding unique characters that people enjoy. Is it guaranteed? No, it obviously isn’t, but it’s VERY likely that we’ll get one whether you like it or not due to it’s current state.