Balance reasons
It is possible that there are 2 packs but lets look at the consequences combined with
18. A werewolf pack wins when all remaining werewolves belong to its pack and the remaining number of villagers is equal or fewer than the number of werewolves.
Usually at a game with 21 pople you have 5 wolves 16 villagers. That allows 5 mislynches to catch 5 wolves.
With 6 wolves (single pack) the village would lose with the 5th mislynch a very hard condition.
With 2 packs and rule 18 the village doesn't lose as long as there is at least 1 villager alive and members of the 2 packs.
Lets asume 2 packs a' 3 wolves. Now the village can do 7 mislynches and still win. If in the szenario only villagers die (8 mislynches 7 nightsnacks) the wolfpack that just happen to hunt at the night after the last villager died wins automatically.
2 packs a 3 wolves that don't know each other but win jointly I can imagine but with rule 18 I doubt that ther are 2 packs.
Of course NobleHelium could well have done it without thinking everything through and not noticing the consequences. In that case he can thank all gods that I'm not a wolf this time else all hell would have broken loose already
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So until I see a different pack-color I rather go with the single pack setup.