Asgard has been devastated by the events of Ragnarok. Surtur scorched the place down and Hela was unable to stop him.
Thor and Loki were forced to release Surtur because by destroying Asgard, Hela’s power was reduced and she could be defeated.
Before his death, Odin had spoken to Loki and Thor that Asgard was not a place but its people. Therefore Thor prefers to save the people of Asgard than Asgard itself.
Valkyrie is now Queen of Asgard because Thor prefers to adventure with the Guardians of the Galaxy exploring outer space.