He did not succeed with his brothers, nephews, he had limited success with is nephew Lot. The word Lecha means that Avraham and Sarah alone should go to Canaan, no other family members should come along. Lot tagged along. Once Lot decided to remain at Avraham's side, Avraham had to teach him and train him, even though he did not want him to come along. In order to be a great teacher one must be able to reach his own family.
Teaching begins at home.
In order for Avraham to be considered the Av Hamon Goyim he had to be able to show success within his family. Terach was his success story. He was respectful of his father and kept in the background to give the impression that it was Terach alone who initiated the decision to leave Ur Kasdim. He knew that Terach, the Baal Teshuva, had attained a very high level and did not want diminish the the great achievement and sacrifice that Terach decided to make. There is a verse in Job, Mi Yiten Mitamay Tahor Halo Echad.