Because he was an Australian, and not a part of the scientific establishment, and because he found the fossil in Africa, and not Europe or Asia, where the establishment looked to for man's origins, his findings were initially dismissed.[2]
Dart's closest ally was Robert Broom whose discoveries of further Australopithecines (and Wilfrid Le Gros Clark's support) eventually vindicated Dart. So much so that in 1947, Sir Arthur Keith said "...Dart was right, and I was wrong."
Brain C.K. Raymond Dart and our African origins, in A century of Nature: twenty-one discoveries that changed science and the world, Laura Garwin and Tim Lincoln, eds.