"Let me go ! I'll never sign ! I'll never sign !"
Zombies are supposed to be dead, hence they are zombies. All responsible culprits in 'COTC' were killed in first movie. How/why sequels ? (<- already regretting asking the question). It's not like Freddy or Jason that can magically reappear over and over after being (to quote Homer Simpson) "unkillable".
CK