For anyone that isn't familiar with the classic 1968 film directed by the late George Romero, Night of the Living Dead never uses the word "zombie," referring to the shuffling, virus-infected living dead as "ghouls" (which is more appropriate, given their hunger for human flesh). The association with zombies largely came from the sequel, Dawn of the Dead (1978), which used "zombie" in place of "ghoul" and which was distributed internationally under the title Zombi (and variations thereof); the Italian release, re-edited by Dario Argento, actually spawned a separate film series under the title Zombi. In addition to which, Romero's successful films sparked a number of imitators - City of the Living Dead, Erotic Nights of the Living Dead, Hell of the Living Dead, and Zombie Holocaust for example all came out in 1980, and were inspired by the release of Dawn of the Dead (1978) and Zombi 2 (1979; the sequel to Zombi, which was the Italian re-edit of Dawn of the Dead).

I don't know if Romero ever gave a specific reason for the shift in "ghouls" to "zombies"; the decade of zombie films released in between Night and Dawn probably helped, but I don't want to get into speculation.