Marsupials like kangaroos are plentiful and diverse in
Australia. Placental mammals (like you) dominate on
other continents. Why?
Australia didn't have placental mammals until
recently.
Where placental mammals evolved, they outcompeted
marsupials.
Marsupials have outcompeted placental mammals in
Australia.
Marsupials evolved in Australia, and very few escaped
to other continents.