I'm not an ammo / gun expert - several here - but the nice thing about the 9mm is that you have more rounds in the clip. Many more.
My shotguns only hold 5 or 6 - one chambered, rest in the magazine.
Granted, a 12 ga. slug packs a bit more punch than a 9mm round.
Especially at close range - like a bear looking for a snack.
The article does mention that both bears were "aggressive" - I wonder if that has anything to do with the time of year up there?
LVM