Natalie Portman previously said she was finished with the MCU

She had issues with Marvel Studios when they were working on The Dark World because they told her they were going to get Patty Jenkins (who became the director of Wonder Woman) to direct, but after got Portman under contract to do the movie, they went with another director. Her cameo in Endgame was footage shot from The Dark World. I imagine by letting her be an actual hero instead of a damsel in distress/love interest it was easy for them to get her to come back.

It’s popular now so she came back.

Throw more money at anyone and they'll come back.

Her specific issues were about her underwritten part. And she definitely said she was done with the whole thing. Now it's clear they bent over backwards to write Jane into the film in a huge way, so they're giving her what she wanted.

Its not like she didn't know what part she was taking when they made Thor 1. She love interested. Not the protagonist. Gwyneth Paltrow been playing that part for 10 years without bitching about it. And why bend over backwards for a character that was a drag on the first two movies.