I get the feeling that the Fascist world conclusion that was cut could've worked as an in-universe mindblower that chronologically occurs around the time of "Ethan's Plan" as intended but that was wiped from our Morty's memory for some convoluted reason and kept in storage to be discovered later (I mean in the show some mindblowers were because rick did something personally embarrassing during an otherwise unrelated adventure so there is precedent)... Actually, now that I think about it that setup could be used to tie up most of the unresolved storylines that were not related to the main story so that may be a decent pitch for the (maybe) Epilogue or part of it.
The Fascist world conclusion had a lot working against it, repeated story beats, a not very good story imo, more art... And, on top of that, every time I went to work on it (write it), I found that I simply didn't want to. Given how close it was to the start, and how much I was put off doing it, removing it was the only was I could move forward.
As for Space Beth, disgust and confusion aside her reaction seems rather cold considering who Morty ends up being
Space Beth is an outsider to the situation, unable or unwilling to accept all that has happened. When she first arrives (in her first scene), she behaves like nothing has changed, that she can gallivant around the galaxy an return home to find everything exactly as it was.
Everyone else got eased into the situation, but Space Beth, it's dumped on her at once. She sees the end of Morty's actions, but not how any of them got there. Her options (in her mind) are to stay and confront it, or run away. And she is the Beth that left, rather than stay with her family.
Or, in short, she is too stubborn to even try to accept what is happening.
Lastly, kind of bummed Tricia isn't even mentioned in passing during the conclusion.
So~ the main reason she isn't there is because she doesn't need to be. She's had her "ending" already, she just doesn't appear in the ending scene. But I can see why people would have liked her to be included one last time. She will appear in the Epilogue (if I ever get there...)
Jessica wasn't planned to be in the end scene either. She was going to stay behind on Unity's ship, rather than come on the Citadel adventure (because already SO many characters). In the end, it felt weird for her to stay behind, so she is just "there" most of the time (with a few good moments along the way). Having Tricia on the adventure too would have been just too much.