Because of the remoteness. Long before the first reptiles, Australia was connected to a giant southern landmass called Gondwana. Two hundred million years ago, about the time dinosaurs came to power, this supercontinent began to break apart, creating new marine barriers between Australia and its neighbouring lands. Although a few links with Gondwana remained, most Australian dinosaurs developed in isolation. You would not see Stegosaurus, Brontosaurus or Tyrannosaurus roaming our ancient landscape. Instead you would find our own unique breeds of Australian dinosaurs, similar - but not identical - to the well known varieties of the northern hemisphere (Allosaurus is an exception to this rule).

     Most Australian dinosaurs are classified with the Ornithischians a varied group of plant eaters (armoured dinosaurs are ornithischians). Fewer in number but greater in size are the gigantic Sauropods or Brontosaurs. Australia had its share of predators as well; these extinct killers are scientifically known as Carnosaurs. All three groups of dinosaurs ruled the land for over 130 million years, ranking them among the most successful vertebrates in Earth's history.

     At first, Australia was like most world climates 200 million years ago - warm and tropical, with low relief and vast swamps. Then continental drift began to carry our land off in a cooler southerly direction. Volcanic eruptions and rift valleys changed the shape of the landscape. By the peak of dinosaur development 100 million years ago, Australia was drifting into the south polar region. We know from the remains of pines, ginkgoes, tree ferns and early flowering plants that this ancient polar climate was not as harsh as modern Antarctica. Even so, any Australian dinosaurs were exposed to seasonal snow and ice - as well as three to six months of winter darkness. 

Information from the poster, The World of Australian Dinosaurs, published by the Gould League of Victoria