re: Why Are Zombies So Popular Now?

Btw Romero didn't refer to his monsters as "zombies" at first.
NOTLD was influenced by The Last Man (featuring Vincent Price in the first film adaptation of Richard Matheson's I Am Legend). The "look and feel" of the films is quite similar.

Romero worked for a local Pittsburg tv station and you see some of the sets in the local news cutaway shots. The newscamera style made the ultra low budget film seem gritty and realistic.

At that time it was required to put the copyright notice on the film and the title was changed. The "c" notice was left off and the film slipped into public domain, costing the producers... lots. But also meant it'd get tons of AirPlay and develop a cult following. Both Night and Dawn are mentioned in the book Cult Movies.

Prior to Romero zombie films featured voodoo and Caribean settings. The undead didn't ear human flesh and were under the control of a witch doctor.

The cause of the infection in Night isn't clearly an infection. It's suggested some strange lunar activity caused the outbreak.

Cult Movies points out the protagonist in Night's refusal to go into the basement is proven to be wrong. Everyone else dies and he is the only survivor (spoiler alert omission) because he goes into the basement.

In times of great uncertainty alien films are usually popular. 1950s had the Cold War, instant threat of complete destruction, fear or communism and UFO films were popular. Expect to see more alien / invasion / post apocalyptic films during the next few years.