Why do some Jehovah's Witnesses choose to be atheist or agnostic?
Because we were disillusioned and we don't want it to happen again.
After I left the WT I spent (or wasted, some could argue) the next 18 years of my life searching for the true religion. I attended many churches and went near-sighted reading the Bible, Apocrypha, Book of Mormon, Calvin, Augustine, Aquinas, ad nauseum.
Once I concluded the Christianity was a total fraud, I looked into Islam (another big scam) and then some of the eastern "religions". I have to admit that Buddhism and Hinduism are actually good forms of psychology if you discount all the supersitious accretions and metaphysical speculation. Lao-tzu and Confucius were great ethical philosophers, but they weren't prophets (nor did they claim to be).
Eventually I just concluded that religion is bunk. I tend to shy away from generalizations, but I can say quite confidently that every person who has ever claimed to receive a revelation from God was either deluded or, more likely, a charlatan. Harsh? Maybe, but keep in mind that I spent EIGHTEEN years of my life in a sincere search for truth and this is the conclusion I've come to.
Religion is, even at its best, an utter waste of time and energy that could be spent much more productively. If anyone hungers for truth then they should apply themselves to philosophy, mathematics, and science (in that order). Pythagoras, Plato, and Liebniz grasped more about the true nature of reality than any theologian. Friedrich Nietzsche's philosophy offers more hope than any apocalyptic eschatology. Science has improved more lives, healed more illnesses, and eliminated more poverty than an army of the most well-meaning saints ever could. Banish faith and embrace reason! Your life can only change for the better.