I think you need to add more requirements to qualify your question. As it stands some answers might be:
1) The bears simply like it there, as the climate does not bother them.
2) There is some highly profitable reason why communities might spring up there. For example: The bear populations of the world are tapping too many food sources. Deer/sheep/cattle is running scarce. These particular bears moved up north and started raising some other cold hardy animal which only thrives up there, and now have a reliable food source that they do not have to share.
3) Living up there for a few years during a certain time of their lives is a rite of passage within their culture. Thus all "teenagers" go there for 2 years when they reach "X" yeas old. The population there is always transient.
4) A great enemy has driven the bears into the cold north, where these other creatures cannot survive.
5) These particular bears are outcasts of bear society, and have simply adapted to the cold. Their community ended up thriving, and becoming quite powerful, which was a surprise to the "civilized" bears. (Think Australia)
Do you see where I'm going with this?