Oh, that's a pretty good idea. I didn't even think about how we could come up with a canon reason for their absence.
Perhaps it was the result of feuding among the Packs, or Velvetstar convinced the others to kick them out because she thought it would be beneficial to her own Pack (since she's probably the most likely to do something like that)? It could also have something to do with the twolegs, like maybe their high-ranking members were captured, which caused the remaining members to disband, and the other three alphas just didn't see attempting to restore two rival groups worth risking their own dogs for.