Here's why:
* Slifer the Sky Dragon is an exclusive card to the physical trading card game. It was not included in any of the digital releases of Yu-Gi-Oh! Eternal Duelist Soul.
* Eternal Duelist Soul focuses on a specific set of cards. The game features cards from various eras of Yu-Gi-Oh! but doesn't include all the cards from the physical game.
While you can't get Slifer the Sky Dragon in Eternal Duelist Soul, you can enjoy playing with other powerful cards and experiencing the nostalgia of older Yu-Gi-Oh! sets.