I believe it's for a few reasons.
One, it gets Will-O-Wisp. This is a tremendously useful move capable of hindering physical attackers severely. Two, it gets Snarl. Snarl hits both opponents and lowers their Special Attack, making it harder for special attackers to perform well.
It's fairly fast and has solid Attack and Special Attack, making it able to run either side well. It gets Extreme Speed and Flare Blitz, allowing it to take down opponents easily.
It also gets the incredibly helpful Ability Intimidate, which further lowers an opponent's Attack. This combined with Will-O-Wisp can all but completely cripple a physical attacker. Arcanine can also be used with Justified. Hitting it with a Dark type move such as Beat Up from a Whimsicott teammate can boost Arcanine's Attack quite high, making it a potential sweeper.
Hope I helped!