Why is it that athiest often (not always) direct their argument for athiesism towards Christians?
It's usually a defensive rather than an offensive tactic. Most of the theists we encounter are Christians. There's little point in me trying to convince you of the absurdity of the Bhagavad Gita when you don't believe in it anyway.
Why not direct your argument towards Buddists or Muslims or even New-agers?
Most of the arguments against the Christian god work equally well against his Muslim equivalent. Buddhist and New Age beliefs tend to be slippier and harder to define and argue against. Often what they believe turns out to be functionally equivalent to atheism, or at least so vague that disagreeing with it is a futile exercise.
Is it because your only point of reference is JWs and the bible and therefore easier to reference?
Partly. I am much more familiar with Christianity (and specifically JW) than I am with any other religion. It's therefore much easier for me to argue points of detail about their specific beliefs. And also, as I mentioned, most of the theists I encounter are Christians anyway.
My arguments against the existence of gods don't ultimately depend on any one holy book, but some people's arguments for their gods do depend on their holy book. It may be necessary to convince someone that there's no reason to believe their holy book is inspired before moving on to the point that there's no reason to believe the universe was created.
Finally, the word "atheist" is an exception to the usual "i before e..." rule, as it's root is the Greek word theos. Or you might remember it better by considering that it's an "-ism". (Excuse the pedantry, it's just such a common error it needs to be highlighted from time to time.)