Why is a koala bear a marsupial and not a bear?

Because it doesn’t have the right koala-fications.

Since this subject has come up, here are some facts about koalas. According to the San Diego Zoo, koalas are mammals, and they do resemble teddy bears with their round, fuzzy ears. But they are definitely marsupials, pouched mammals whose group includes kangaroos, wallabies, wombats, and opossums. They look soft and cuddly, but their fur feels more like coarse sheep’s wool. They are naturally solitary and most active at night. Koalas eat only eucalypt leaves, which are difficult to digest, so they spend most of their time napping and eating. You can find them living in the forests of southeastern and eastern Australia. (https://animals.sandiegozoo.org/animals/koala)

To watch koalas, the San Diego Zoo has a Koala Cam that streams during San Diego’s daylight hours. Once it gets dark, that day’s stream is rebroadcast. (https://zoo.sandiegozoo.org/cams/koala-cam) And remember, they are most active at night.