Why only marsupials in Australia?
125 million years ago Gonwana was broken up in three parts:
a) South America /Africa,
The first marsupials came from Laurasia (the big landmass of the northern hemisphere) and spread to South America. In this time there was a connection from the southern most South America to Antarctica/Australia. Later on, the placental mammals went nearly the same way, but at this time the connection of the South American and Antarctic/Australian landmass did not exist anymore. The placental mammals of this time were more competitive and most of the marsupials died off. Finally the Antarctic continent and the Australian landmass broke up – the Antarctic moved to the South Pole and became a very cold climate situation. South America one more time joined the north American continent (isthmus of Panama) that means a new invasion of very competitive placentals.