Because Noelle Stevenson either can't or doesn't know how to manage a writing staff.
It seemed like she told the other writers to make Catra a villain then they left,
and she didn't care what they wrote and pushed ahead with her idea of a romance anyway.
I wouldn't be surprised if Noelle was too busy with tweeting "background lore" to pay attention,
to what the other writers were doing.
Noelle wanted a Korrasami, bubbline ending so she wouldn't have to put in any effort into
actually writing her catgirl x magicgirl lazy anime fanfic level of writing.
Honestly though she really shouldn't have a happy ending even with the whole Adora forgives her.
Micah is going to either send her to jail or just vaporize her for basically
killing his wife, the queen, and Glimmers mom(who should still hate her).