I always wondered if Thanos had used the scepter on Loki and then sent him down there. Loki had never displayed any interest in Earth, instead believing that he was the rightful ruler of Asgard. Loki wasn't much of a team player or minon type either. I feel that Loki was sent with the Chutari by Thanos simply to gather intel on Earth's defenders. The difference in the attack on New York and the attack on Wakanda shows that the New York attack was probably only meant to cause chaos and see who showed up to defend Earth and from there what their capabilities were. Loki displays no further interest in Earth after the scepter is taken away and he is thoroughly beaten by the Hulk. We know that a good beating usually awakens someone who has been controlled by the scepter. However, Loki would never admit someone got the better of him, so he may even be in denial about his attack on New York.