The concept of fleeing implies an effort to escape from a danger or a threat, and so assumes you can find a place where you are safe from the danger (perhaps because the adversary cannot find you). Earth and sky/heaven fled to escape the danger implicit in God’s judging presence but could find no place to hide and/or escape the danger. The God on the throne, after all, is so great as to fill heaven and earth (Jeremiah 23:24), and so there was no escaping him. The implication is twofold:
The throne (and its divine Occupant) is the only stable and permanent entity in the world.
No resident on earth or in heaven can escape from (the Judge on) the throne.
11 Then I saw a great white throne and him who was seated on it. From his presence earth and sky fled away, and no place was found for them.