>>54331134J-Sneasel are cunning in nature and could function very well as ambush predators under the cover of darkness whereas their Hisuian counterparts seem to be largely diurnal. They could easily have survived in hidden pockets in the Alabaster Icelands or Coronet Highlands and proliferated in secret, at which point their numbers would grow too great to be stopped.
Assuming H-Sneasel lacks a particularly mean spirited nature it's possible the savagery of J-Sneasel allowed them to function better in the harsh environment of Hisui.
