The Motion Picture just dropped you in and explained precisely dick. Same with NuTrek. It didn't tell you anything except how it was DIFFERENT.

(incidentally, I'm still angry that they're calling the NuTrek the 'Kelvin Timeline' when multiple timelines are *explicitly* not how time travel works in Star Trek. There is only ONE timeline, and time travelers are exempt from the effects of the changes. The changes they make HAPPEN and overwrite the original world. Did nobody on the production team watch City on the Edge of Forever? Or any of the other dozen or so time travel episodes that all confirm this exact thing?)

(For that matter, it's clear that there was ALREADY changes considering the state of the ship Kirk's dad was on was clearly far more advanced than that of even a new Refit Connie. The entire premise is bullshit! And I'm a fool for seeing all three NuTreks to date in theatre and being part of the problem...)