Lots of good comments on this thread--in particular, those of GaryBuss and Iquit.
This may sound strange, and it certainly doesn't apply to every JW, but I think that a lot of Witnesses are actually not very religious. I know I wasn't, and the same with most of my JW friends. JW-ism is much more like a job than a religion. You attend business meetings, make sales calls, and turn in monthly time sheets. (GaryBuss points this out quite often on his threads). There's lots of office politics, and you get "promoted" if you have good numbers...and brown-nosing the bosses helps, too.
And when it comes to theology, lots of Witnesses self-identify based on what they don't believe (trinity, hell, soul) than what they do believe. JWs (at least the ones I know) love to mock "born-agains" and ridicule their teachings as stupid and contradictory (e.g., how can Jesus be his own Father?). This leaves former JWs primed to become secular humanists. To paraphrase Richard Dawkins, the JWs were already atheists about lots of gods (including the Trinitarian god of Christendom)...now they're just going one god further.