The new creators of the sequeals didn't add them.

They do physically appear in Wings Of Change, they just don't have any lines of dialogue.

There is no definite answer. The sequels were directed and animated by different creators. For whatever reason why they didn't add the other characters is up to them. There are theories thought.
For the second movie: Wolf Quest, it focused mainly on Balto's wolf heritage and the wilderness, so there would be no need for the other characters since they reside in Nome.
The third movie is a but tricky to fully answer. Possibly, they have no need for these characters any longer and didn't include them. They could also be trying to focus on Kodi and the plan situation more.

they had roles that suited only the movies they were in. also alot of people belive they were just too lazy to put them in because they would just be more chars to draw and stuff