Regardless of whether you are believer or not, the fact that the Jesus of the WTS is VERY different than the Jesus taught by most of Christendom (and yes, they use this term in a positive manner unlike what JW's have been taught!). I would suggest that you do a little research on the matter, but a couple of the big differences have to do with the deity of Jesus (God or "a god") and the definition of Grace (or in JW talk, "underserved kindness"). Again, whether you believe or not, you can see why people are drawn to something presented in a very different light than what they were taught as JW's.
Some may become Atheists after leaving the WTS, but it is not unreasonable to compare it to food. Some may have an epiphany and choose to be vegetarians, while others may just give up red meat and switch to chicken and fish.