For most species of mammals, the primary function of the tail is communicative. The position and movements of the tail convey emotional states and intentions to fellow members of the species.
Bears are highly solitary animals, rarely tolerating the presence of other bears, with the exception of a mother bear with her cubs or during the brief mating season when a male and female come together.
They don’t have a strong need for social communication means. In this lifestyle, the length of the tail is not influenced by natural selection, and as a result, their tails may become reduced in size. Bears are large and powerful animals, and their bodies are adapted for their particular needs, such as foraging, hunting, and hibernating. In the case of tails, a shorter tail might be more practical and less prone to injury in their environment and activities.