Kind of like saying what's with this slasher obesseion during the 80's and 90's.
My opinion is that zombie films have always had staying power and fandom on some level. In the 60's to mid 70's it was the George A. Romero films. After that it was the Italian zombie gore films. Then comedy was fused with Return of the Night of the Living Dead, Evil Dead (deadites are possessed humans, but very similar) and Braindead (Peter Jackson). The 90's probably wasn't a great decade for big zombie films, but Resident Evil, 28 ______ Later, Shaun of the Dead and Snyder's Dawn of the Dead; as well as the literature from Walking Dead and World War Z revived the genre.
Take into account that it's now been nearly 50 years of the modern zombie films, there are alot of film makers influenced by the genre (from Raimi, Snyder, Wright, Jackson, etc) and the genre has a bigger demographic (Romantic thriller Warm Bodies and the film, and the comedy Zombieland, and the action thriller film based on World War Z, video games (which have used the genre for decades), and having a well recieved show based on the Walking Dead TV series, it shows people are still interested in it.
Why? It's all subjective. For me, it's the complete transformation and break down of society that I like. I also like how the monsters are humanized, but at the same time its just some relentless evil without a reason (unlike with some other current horror films where the antagonist is given a backstory on why he is evil, so he really isn't "evil").
It's the fact that you have to come to a decision that your family might not be your family anymore and have to shoot them; playing with your morals and ethical shit and what not.
It's also the survival aspect, about how you can live so equipped and be so prepared for months and within an instant you could be dead and eaten.