Why was a bear sacred to Artemis?

On two occasions bears are associated with the wrath of Artemis: of Kallisto, a maiden of Artemis who was seduced by Zeus, and when it was discovered, changed into a bear.

Polyphonte, also a companion of Artemis, scorned Aphrodite in favor of Artemis, so Aphrodite made her fall in love with a bear and drove her made, Artemis caught her mating with the bear and turned all animals against her (the bear killed Polyphonte).