The question refers to the end of Stranger Things Season 3 and the start of Season 4, when Will Byers and Eleven leave Hawkins with the Byers family.
Story reason
Joyce decides that Hawkins is too dangerous after everything that has happened to her family: Will’s abduction and possession, Bob’s death, and then Hopper’s apparent death at the Starcourt Mall. To protect her sons and Eleven from further trauma and supernatural threats, she chooses to move them away and try to start a new life in California.
Eleven specifically
With Hopper believed dead, Eleven has no stable guardian in Hawkins, and Joyce is the adult who best understands her situation and powers. Joyce essentially takes her in as part of the family, so Eleven leaves with Joyce, Will, and Jonathan both for safety and because she now belongs with them.
In-universe motivation
Within the show’s world, the move is meant to get the kids away from the repeated disasters centered on Hawkins and the Upside Down. It also reflects a hope that distance and a “normal” environment will help them heal, even though later events in California show that the past and the Upside Down still follow them.
