Here's why:
* Marsupials are mammals that give birth to relatively undeveloped young that complete their development in a pouch on the mother's abdomen. Examples include kangaroos, koalas, and opossums.
* Grizzly bears, like all bears, are placental mammals. This means their young develop inside the mother's uterus, receiving nourishment through a placenta, and are born relatively developed.
So, while both grizzly bears and marsupials are mammals, they belong to different sub-classes based on their reproductive strategies.