I think it's because basically JWs like certainty, and many ex-JWs simply convert the certainty of knowing there is a God who will soon provide paradise into the certainty that there is no God and they have to provide their own meaning in life.
The more difficult and courageous path is to explore and get to grips with the limits of what we can know and the uncertainty involved in all sorts of human knowledge.
There is also the aspect that JWs enjoy the sensation of thinking they know things other people don't know. They have special knowledge from God. There's a similar tendency among some atheists and self-proclaimed rationalists, or "brights" as some have modestly labelled themselves. So from that point of view you can see why strident atheism is psychologically appealing to many ex-JWs. Although the content of the belief has changed, they still get to perceive themsleves as knowing better than other people.