Here's why:
* Pokémon Yellow has a limited movepool for each Pokémon. Unlike later games, the movepool in Pokémon Yellow is significantly smaller.
* Arcanine's learnset in Pokémon Yellow does not include Flamethrower. Arcanine can learn other Fire-type moves like Ember and Fire Blast, but not Flamethrower.
If you're playing Pokémon Yellow and want a powerful Fire-type move for Arcanine, Fire Blast is your best bet.