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Triple whammy of the anime, games, and trading cards, in that order.
It helped that the world was just coming down from the virtual pet craze started by Tamagotchi and Furby and Pokemon was thematically related to the most recent craze among kids in that you had your own team of pet monsters, also there was Pokemon's own ( admittedly weak ) forray into virtual pets with the Pokemon Pikachu. Basically just a transition into the next big thing.
That and 4kids and Nintendo legitimately did their damndest to market the thing. Mostly 4kids at first but Nintendo noticed that it wasn't the flop they were expecting it to be in the west pretty quickly.
