I think because “I am a Jedi” feels like it’s more appropriate as a conclusion to Rey’s story across the trilogy. She’s got her abandonment as a child and lonely life, her lessons with Luke, and the Kylo Ren and darkside temptations, and declaring herself a Jedi brings all of those together into her taking bold ownership of her position in the galaxy.

That’s why the change is accompanied by other changes to Palpatine’s preceding dialogue, where his lines are reshaped into effectively taunting her that she’s nothing.

As a lesser consideration, another factor is that “All the Sith”/“All the Jedi” feels a bit like it’s a cool thing to say but doesn’t quite hold up to scrutiny, and like she’s ONLY (or mainly) strong in the moment because she’s channeling all of the Jedi. It’s kind a comparison between a cool statement in the moment versus something that better ties together her arc across the trilogy.