Here's why:
* Marsupials are mammals that give birth to relatively undeveloped young which then complete their development in a pouch. Wallaroos are known for their pouches and are a type of kangaroo, which is a classic example of a marsupial.
* Monotremes are mammals that lay eggs. Examples include the echidna and the platypus.
So, while both wallaroos and monotremes are mammals, they are distinct groups with different reproductive strategies.