Exactly. Atmosphere. That's why they use the same genre.
Yes, obviously you're not going to play certain songs as they wouldn't fit into zombies. But there are certainly songs and different genres out there that would fit in.
Seriously whats wrong with some diversification?
For you to say "horrible heavy metal" it looks like you want to pick a fight arguement with people that listen to heavy metal which is just wrong and probably the worse thing you can do imo. Have you ever been to a metal concert and been up front? Obviously not
Just google it. LOL
They don't want people to dance with zombies. They want you to slay them. That's why metal fits and other genres don't.