Dissidia was killed outside of Japan because of huge amounts of Japanese exclusive content. I wanted to see Rinoa in KH, 14 years after the KHII tease. It's ultimately a sign that Kingdom Hearts will be a Disney only game series going forward.
Well, I wouldn't say it's an ultimate sign because Nomura and Yasue seem really shocked that people felt the loss of the FF characters in KH3. Which, they honestly shouldn't be, because even the staff apparently got mad at Nomura for not including them.
Also, the reason why Dissidia was killed outside of Japan is also due to the battle system and most thought it was going to be similar to the PSP version on steroids. Instead, it's 3v3 and pretty clunky, and also lacks a story like the previous entries did. It's not a bad game, but there was a lot to be desired. There's also the fact that it was lifted off from arcade systems, so to begin with, Dissidia wasn't even really meant for consoles. If they had developed it for the PS4 from the ground up, this would've been a completely different game, but they can still bounce back.
In BBS's case, it was basically the same reason they weren't in KH3. Even though they planned on including the young RG gang, they didn't want to distract from the main story and decided to leave them out. It wasn't like they weren't even considered to begin with.
Which at least shows that Nomura's consistent with his reasoning for these kinds of things. Not that I agree, but it does show that it hasn't happened the first time in KH3, if that's any consolation.
Because BbS sucks as a prologue in almost every way? Why don't we see the downfall of Hollow Bastion or the experiments or the first Heartless or the world's walls being broken in BbS? Why don't we see a callback to Triton learning about the Keywielders in Atlantica or Genie having heard of the Keyholes before in Agrabah or Xehanort's offscreen interaction with Maleficent where she apparently learned everything? I’ve never had the same expectations for the crappy side games as the main titles.
If I remember correctly, didn't the fall of Radiant Garden happen one year after BBS? If that's the case, then that would explain why we didn't see that happen. We sorta get a bit of the experiment stuff with Lea and Isa (which we now know ALSO ties into Subject X) for a couple seconds, but maybe a lot of this didn't kick into high gear until after Apprentice Xehanort. As for Triton, this honestly could've been something he witnessed WAY before BBS even occurred, it's also possible that the Age of Fairy Tales is royal knowledge that's passed down as well, as ruler and protector of the world. Genie is a cosmic being, so he could've come across that knowledge so many years ago as well.
I do wish that BBS covered more of this though, I personally would've wanted more insight into Mickey's training and even stuff like where Terra and Aqua originally came from. I think there were some rewrites that happened though, because in the Jump Festa 2008 trailer, it's almost completely different than the version we got. Ven saved Terra somehow, Master Xehanort was in Olympus Coliseum, Terra fought Dragon Maleficent, Terra and Master Xehanort facing off in the Great Maw apparently... And I like to think that a lot of what you were looking for would've been covered in BBS Vol. 2, but unfortunately I don't think we'll ever get that now.