Why didn't Doctor Strange appear when Hela came to Earth?
Earlier in the latter film, when Thor meets Strange for the first time, the dialogue reads:
Strange: “So I keep a watch list of individuals and beings from other realms that may be a threat to this world. Your adopted brother Loki is one of these beings.”
Thor: “He's a worthy inclusion.”
Strange: “Then why bring him here?”
Thor: “We're looking for my father.”
Strange: “So. If you were to tell you where Odin was, all parties concerned would promptly return to Asgard?”
Thor: “Promptly”
Strange: “Great. Then I'll help you.”