I think it's a fairly common trend in the genre, it's not restricted to zombies either, in The Passage the word (possible spoiler) vampire is said maybe once and while there are similar terms used there seems to be a reluctance to use the commonly recognised word. I suspect that writers feel that there are too many connotations attached or that they won't be taken seriously if they straight up admit they're using these creatures (look at how many games get dismissed because they have zombies in them?).|
There have been a few approaches taken:
1. The alternate reality: A parallel universe where nobody has heard of zombies.
2. The denial: "Don't even say the 'Z' word!" where the characters refuse to come to terms with the situation.
3. The standard re-naming: Walking Dead's 'walkers' being a prime example.
Portable view - Never forget.