Well that's because the maps in Zombie mod change the gameplay much more than any of the other servers.
Then of course your average zombie mod player usually has a personal map preference; either escape, cade, maps made after movies (LOTR, Indiana Jones...) or maps with 'cool' sounding names (Ninja_beta). It can be a pain in the ass finding a map that will shut up most of these guys up or else prepare to get spammed with "This map sucks! RTV!"