Actually, it was never intended to be a sequel. It was just seen that way because of the naming fiasco over in Europe. It was released as "Zombie 2" in most of Europe (as "Voodoo, Schreckensinsel der Zombies" (Voodoo, Horror Island of Zombies) in Germany, and "Zombie Flesh Eaters" in the UK) due to the popularity of Dawn of the Dead, which was released as "Zombie" by Argento. So they wanted to capitalize on that.