Zombies are popular IMO for several reasons.
1) Fits the atheistic world view of today that other monsters (vampires, demons etc.) fail to fit in by being too supernatural.
2) Luddite fear of science going too far, creating a bio-weapon with severe backlash in a global pandemic.
3) Plausibility. Zombies are a dumb threat in which the lazy average Joe can be heroic.
I think also that zombies are hard to personalize. More at dehumanized.